Fix re match for engine test_if

Led run: https://ci.chromium.org/raw/build/logs.chromium.org/flutter/led/keyonghan_google.com/950806e381157cd280919d4f14a38d6784092f480de32b2ab30f9dfe41d0c439/+/build.proto?server=chromium-swarm.appspot.com

Change-Id: I4032ab683cb3ff6f36304a0f32a17ed8a0e6ed8c
Reviewed-on: https://flutter-review.googlesource.com/c/recipes/+/54644
Reviewed-by: Godofredo Contreras <godofredoc@google.com>
Commit-Queue: Keyong Han <keyonghan@google.com>
diff --git a/recipes/engine_v2/builder.py b/recipes/engine_v2/builder.py
index 3c8e113..32c317a 100644
--- a/recipes/engine_v2/builder.py
+++ b/recipes/engine_v2/builder.py
@@ -98,8 +98,8 @@
 def _should_run_test(test, branch):
   """Whether the current test should on this branch."""
   # Default to wildcard to run tests everywhere.
-  test_if = test.get('test_if', '.*')
-  regex = re.compile(test_if)
+  test_if = test.get('test_if', '')
+  regex = re.compile(f'.*{test_if}')
   return regex.match(branch)