[test] Support LC_ALL=en_US.UTF-8 in test runner

Currently, `intl/regress-7770` fails on environments with `LC_ALL`
set, e.g.

    export LC_ALL=en_US.UTF-8

While engineers can manually work around it using `unset LC_ALL`
before running the test suite, it would be more convenient if the test
runner didn't rely on the absence of this environment variable in the
first place.

Bug: v8:8845
Change-Id: I8116e2fd369be1d561dfe465f2901d07d3f75510
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1680538
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Tamer Tas <tmrts@chromium.org>
Auto-Submit: Mathias Bynens <mathias@chromium.org>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62417}

test/intl/testcfg.py

@@ -58,6 +58,9 @@ class TestCase(testcase.D8TestCase):
 
   def _parse_source_env(self, source):
     env_match = ENV_PATTERN.search(source)
+    # https://crbug.com/v8/8845
+    if 'LC_ALL' in os.environ:
+      del os.environ['LC_ALL']
     env = {}
     if env_match:
       for env_pair in env_match.group(1).strip().split():