[test] Fix wrapper order in verify-predictable mode

If a command prefix should be used in predictable mode, it should
wrap around d8 and not around the predictable wrapper.

NOTRY=true
TBR=majeski@google.com

Bug: v8:7166
Change-Id: I16f33090c647ea4f6f040c6194908df6500b82a3
Reviewed-on: https://chromium-review.googlesource.com/832206
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50160}

tools/testrunner/standard_runner.py

@@ -283,8 +283,8 @@ class StandardTestRunner(base_runner.BaseTestRunner):
         options.extra_flags.append("--verify_predictable")
         options.extra_flags.append("--no-inline-new")
         # Add predictable wrapper to command prefix.
-        options.command_prefix.append(sys.executable)
-        options.command_prefix.append(PREDICTABLE_WRAPPER)
+        options.command_prefix = (
+            [sys.executable, PREDICTABLE_WRAPPER] + options.command_prefix)
 
       # TODO(machenbach): Figure out how to test a bigger subset of variants on
       # msan.