Revert "Fix flatbuffers_unittests failure-mode"

This reverts commit 108146f8.

Reason for revert: See https://bugs.chromium.org/p/chromium/issues/detail?id=938020#c6

Original change's description:
> Fix flatbuffers_unittests failure-mode
> 
> This is failing on Win7 Tests (dbg)(1):
> https://ci.chromium.org/p/chromium/builders/ci/Win7%20Tests%20%28dbg%29%281%29?limit=200
> 
> Failures are causing python errors because the "failures" element must be a list.
> The actual cause of the failure is not yet known.
> 
> Bug: 938020
> Change-Id: I53742dbf49f97a8828af5675a0360e431ed376c8
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1503614
> Reviewed-by: Dirk Pranke <dpranke@chromium.org>
> Commit-Queue: Charlie Harrison <csharrison@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#637839}

TBR=dpranke@chromium.org,csharrison@chromium.org

Change-Id: Iebb35ee8e42272f742497b25713d0927fb00ff14
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 938020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1504597Reviewed-by: Darren Shen <shend@chromium.org>
Commit-Queue: Darren Shen <shend@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637997}

testing/scripts/run_flatbuffers_unittests.py

@@ -40,16 +40,15 @@ def main():
     exe = os.path.join('.', 'flatbuffers_unittests')
 
   env = os.environ.copy()
-  failures = []
   with common.temporary_file() as tempfile_path:
     rc = xvfb.run_executable([exe], env, stdoutfile=tempfile_path)
 
     # The flatbuffer tests do not really conform to anything parsable, except
     # that they will succeed with "ALL TESTS PASSED".
     with open(tempfile_path) as f:
-      output = f.read()
-      if output != "ALL TESTS PASSED\n":
-        failures = [output]
+      failures = f.read()
+      if failures == "ALL TESTS PASSED\n":
+        failures = []
 
   with open(args.isolated_script_test_output, 'w') as fp:
     json.dump({'valid': True,'failures': failures}, fp)