Add test for issue 3987

Closes 3987

It's unclear which commit resolved this issue, but it can no longer be
reproduced.

tests/source/issue-3987/format_macro_bodies_true.rs

@@ -0,0 +1,26 @@
+// rustfmt-format_macro_bodies: true
+
+// with comments
+macro_rules! macros {
+    () => {{
+        Struct {
+            field: (
+                42 + //comment 1
+                42
+                //comment 2
+            ),
+        };
+    }};
+}
+
+// without comments
+macro_rules! macros {
+    () => {{
+        Struct {
+            field: (
+                42 +
+                42
+            ),
+        };
+    }};
+}

tests/target/issue-3987/format_macro_bodies_false.rs

@@ -0,0 +1,26 @@
+// rustfmt-format_macro_bodies: false
+
+// with comments
+macro_rules! macros {
+    () => {{
+        Struct {
+            field: (
+                42 + //comment 1
+                42
+                //comment 2
+            ),
+        };
+    }};
+}
+
+// without comments
+macro_rules! macros {
+    () => {{
+        Struct {
+            field: (
+                42 +
+                42
+            ),
+        };
+    }};
+}

tests/target/issue-3987/format_macro_bodies_true.rs

@@ -0,0 +1,21 @@
+// rustfmt-format_macro_bodies: true
+
+// with comments
+macro_rules! macros {
+    () => {{
+        Struct {
+            field: (
+                42 + //comment 1
+                42
+                //comment 2
+            ),
+        };
+    }};
+}
+
+// without comments
+macro_rules! macros {
+    () => {{
+        Struct { field: (42 + 42) };
+    }};
+}