Add regression test for spacer block breakage in themes without spacing.units support

[fullofcaffeine]

What problem does this address?

Follow-up to: #37774

The core spacer block broke in themes on which the useSetting function returned false for the spacing.units settings. A regression test would make sure the same problem doesn't happen again in the future.

What is your proposed solution?

There are two approaches to testing this:

1. An E2E test that changes to a theme, inserts the block on the editor, and makes sure it doesn't error;
2. An unit test that stubs useSetting('spacing.units') to return false and makes sure the component renders without errors.

I tend to think the E2E is better as it reproduces what users saw and covers more ground, potentially, and doesn't require stubbing values, but I'm open to hearing your thoughts about it!