[dart2js] Rename issue-specific tests.

The `regress` folder in tests/web and tests/web_2 has been renamed to
`issue`. All issue-numbered tests have been placed there; other tests
have been moved out of that folder.

Change-Id: I4534d34b2ef21f7accc45c3ce097d9be74845e4d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/235984
Reviewed-by: Joshua Litt <joshualitt@google.com>
Commit-Queue: Mayank Patke <fishythefish@google.com>

tests/web/regress/issue/32853_test.dart

@@ -6,10 +6,9 @@
 
 // Regression test for issue 32853.
 
-
 int foo<T extends Comparable<T>>(T a, T b) => a.compareTo(b);
 
 main() {
   int Function<T extends Comparable<T>>(T, T) f = foo;
   print(f<num>(1, 2));
-}
+}