[unittests] initialize the loop variable

This CL fixes the UnboundLocalError in unittests testsuite.

R=machenbach@chromium.org

Bug: chromium:1091200
Change-Id: I9a4e032915b2750c28e3eb9f97042b75ca547801
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2232540
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Auto-Submit: Tamer Tas <tmrts@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68203}

test/unittests/testcfg.py

@@ -48,6 +48,7 @@ class TestLoader(testsuite.TestLoader):
     # TODO create an ExecutableTestLoader for refactoring this similar to
     # JSTestLoader.
     test_names = []
+    test_case = ''
     for line in output.stdout.splitlines():
       test_desc = line.strip().split()[0]
       if test_desc.endswith('.'):