test: Fix DateTimeRangePicker unit test failure (#11502)

## Description 📝

This attempts to fix a unit test failure in `DateTimeRangePicker.test.ts` when the test is run in an environment whose system locale is not set to central time (UTC-06:00). Also does some clean up related to the way that the system time is mocked.

## Changes  🔄

- Fix test failure in DateTimeRangePicker unit tests
- Clean up

## How to test 🧪

```bash
yarn test DateTimeRangePicker.test
```

packages/manager/src/components/DatePicker/DateTimeRangePicker.test.tsx

@@ -27,14 +27,12 @@ const Props: DateTimeRangePickerProps = {
 };
 
 describe('DateTimeRangePicker Component', () => {
-  let fixedNow: DateTime;
-
   beforeEach(() => {
     // Mock DateTime.now to return a fixed datetime
-    fixedNow = DateTime.fromISO(
+    const fixedNow = DateTime.fromISO(
       '2024-12-18T00:28:27.071-06:00'
-    ) as DateTime<true>;
-    vi.spyOn(DateTime, 'now').mockImplementation(() => fixedNow);
+    ).toUTC() as DateTime<true>;
+    vi.setSystemTime(fixedNow.toJSDate());
   });
 
   afterEach(() => {
@@ -51,8 +49,7 @@ describe('DateTimeRangePicker Component', () => {
   });
 
   it('should call onChange when start date is changed', async () => {
-    const currentYear = new Date().getFullYear(); // Dynamically get the current year
-    const currentMonth = String(new Date().getMonth() + 1).padStart(2, '0'); // Get current month (1-based)
+    vi.setSystemTime(vi.getRealSystemTime());
 
     renderWithTheme(<DateTimeRangePicker onChange={onChangeMock} />);
 
@@ -62,11 +59,17 @@ describe('DateTimeRangePicker Component', () => {
     await userEvent.click(screen.getByRole('gridcell', { name: '10' }));
     await userEvent.click(screen.getByRole('button', { name: 'Apply' }));
 
+    const expectedStartTime = DateTime.fromObject({
+      day: 10,
+      month: DateTime.now().month,
+      year: DateTime.now().year,
+    }).toISO();
+
     // Check if the onChange function is called with the expected  value
     expect(onChangeMock).toHaveBeenCalledWith({
       end: null,
       preset: 'custom_range',
-      start: `${currentYear}-${currentMonth}-10T00:00:00.000-06:00`,
+      start: expectedStartTime,
       timeZone: null,
     });
   });
@@ -164,8 +167,8 @@ describe('DateTimeRangePicker Component', () => {
     await userEvent.click(last24HoursOption);
 
     // Expected start and end dates in ISO format
-    const expectedStartDateISO = fixedNow.minus({ hours: 24 }).toISO(); // 2024-12-17T00:28:27.071-06:00
-    const expectedEndDateISO = fixedNow.toISO(); // 2024-12-18T00:28:27.071-06:00
+    const expectedStartDateISO = DateTime.now().minus({ hours: 24 }).toISO(); // 2024-12-17T00:28:27.071-06:00
+    const expectedEndDateISO = DateTime.now().toISO(); // 2024-12-18T00:28:27.071-06:00
 
     // Verify onChangeMock was called with correct ISO strings
     expect(onChangeMock).toHaveBeenCalledWith({
@@ -191,8 +194,8 @@ describe('DateTimeRangePicker Component', () => {
     await userEvent.click(last7DaysOption);
 
     // Expected start and end dates in ISO format
-    const expectedStartDateISO = fixedNow.minus({ days: 7 }).toISO();
-    const expectedEndDateISO = fixedNow.toISO();
+    const expectedStartDateISO = DateTime.now().minus({ days: 7 }).toISO();
+    const expectedEndDateISO = DateTime.now().toISO();
 
     // Verify that onChange is called with the correct date range
     expect(onChangeMock).toHaveBeenCalledWith({
@@ -218,8 +221,8 @@ describe('DateTimeRangePicker Component', () => {
     await userEvent.click(last30DaysOption);
 
     // Expected start and end dates in ISO format
-    const expectedStartDateISO = fixedNow.minus({ days: 30 }).toISO();
-    const expectedEndDateISO = fixedNow.toISO();
+    const expectedStartDateISO = DateTime.now().minus({ days: 30 }).toISO();
+    const expectedEndDateISO = DateTime.now().toISO();
 
     // Verify that onChange is called with the correct date range
     expect(onChangeMock).toHaveBeenCalledWith({
@@ -245,8 +248,8 @@ describe('DateTimeRangePicker Component', () => {
     await userEvent.click(thisMonthOption);
 
     // Expected start and end dates in ISO format
-    const expectedStartDateISO = fixedNow.startOf('month').toISO();
-    const expectedEndDateISO = fixedNow.endOf('month').toISO();
+    const expectedStartDateISO = DateTime.now().startOf('month').toISO();
+    const expectedEndDateISO = DateTime.now().endOf('month').toISO();
 
     // Verify that onChange is called with the correct date range
     expect(onChangeMock).toHaveBeenCalledWith({
@@ -271,7 +274,7 @@ describe('DateTimeRangePicker Component', () => {
     const lastMonthOption = screen.getByText('Last Month');
     await userEvent.click(lastMonthOption);
 
-    const lastMonth = fixedNow.minus({ months: 1 });
+    const lastMonth = DateTime.now().minus({ months: 1 });
 
     // Expected start and end dates in ISO format
     const expectedStartDateISO = lastMonth.startOf('month').toISO();