[test] Create commands after sharding.

Bug: v8:6917
Change-Id: I3a79a7f273df566035cee6a0e22be2e02d67d946
Reviewed-on: https://chromium-review.googlesource.com/808505
Commit-Queue: Michał Majewski <majeski@google.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49865}

tools/testrunner/standard_runner.py

@@ -451,12 +451,12 @@ class StandardTestRunner(base_runner.BaseTestRunner):
         if options.warn_unused:
           s.WarnUnusedRules(check_variant_rules=True)
         s.FilterTestCasesByStatus(options.slow_tests, options.pass_fail_tests)
+        s.tests = self._shard_tests(s.tests, options)
 
         for t in s.tests:
           t.flags += s.GetStatusfileFlags(t)
           t.cmd = s.GetCommand(t, ctx)
 
-        s.tests = self._shard_tests(s.tests, options)
         num_tests += len(s.tests)
 
       if options.cat: