[test] Fix swarming shard distribution

The environment variables for swarming shards are leaking into the gtest runs, which read them as well and in turn skip some tests. Now we make sure those environment variables aren't passed to the subprocesses. 

BUG=v8:5956

Change-Id: I9c93b1facc703a10a88e633074977743ccd24eb0
Reviewed-on: https://chromium-review.googlesource.com/441745Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43161}

tools/testrunner/local/commands.py

@@ -26,6 +26,7 @@
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 
+import os
 import subprocess
 import sys
 from threading import Timer
@@ -62,11 +63,19 @@ def RunProcess(verbose, timeout, args, **rest):
     prev_error_mode = Win32SetErrorMode(error_mode)
     Win32SetErrorMode(error_mode | prev_error_mode)
 
+  env = os.environ.copy()
+  # GTest shard information is read by the V8 tests runner. Make sure it
+  # doesn't leak into the execution of gtests we're wrapping. Those might
+  # otherwise apply a second level of sharding and as a result skip tests.
+  env.pop('GTEST_TOTAL_SHARDS', None)
+  env.pop('GTEST_SHARD_INDEX', None)
+
   try:
     process = subprocess.Popen(
       args=popen_args,
       stdout=subprocess.PIPE,
       stderr=subprocess.PIPE,
+      env=env,
       **rest
     )
   except Exception as e: