Don't check formatting of Front-End test cases

Change-Id: Ie3e55422dc891f3ba37df887f5c96751669dd4cc
Reviewed-on: https://dart-review.googlesource.com/c/90224
Commit-Queue: Peter von der Ahé <ahe@google.com>
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Auto-Submit: Peter von der Ahé <ahe@google.com>
Reviewed-by: Kevin Millikin <kmillikin@google.com>

PRESUBMIT.py

@@ -22,6 +22,8 @@ def _CheckFormat(input_api, identification, extension, windows,
   upstream = input_api.change._upstream
   unformatted_files = []
   for git_file in input_api.AffectedTextFiles():
+    if git_file.LocalPath().startswith("pkg/front_end/testcases/"):
+      continue
     filename = git_file.AbsoluteLocalPath()
     if filename.endswith(extension) and hasFormatErrors(filename=filename):
       old_version_has_errors = False