Blacklist tests due to optimizing toplevel with --always-opt.

Those two tests fail in gc-stress and custom snapshot (embedding mjsunit.js).
This is likely due to different GC timing with the custom snapshot.

R=mstarzinger@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1078653002

Cr-Commit-Position: refs/heads/master@{#27688}

test/mjsunit/mjsunit.status

@@ -106,7 +106,9 @@
   'debug-step-3': [PASS, NO_VARIANTS],  # flaky in no-snap mode.
   'debug-stepframe-clearing': [PASS, NO_VARIANTS],  # only in no-snap debug.
   'debug-stepin-call-function-stub': [PASS, NO_VARIANTS],  # only in no-snap debug.
-  'regress/regress-3717':[PASS, NO_VARIANTS],  # only in no-snap mode.
+  'regress/regress-3717': [PASS, NO_VARIANTS],  # only in no-snap mode.
+  'regress/regress-2451': [PASS, NO_VARIANTS],  # with custom snapshot and gc-stress.
+  'debug-multiple-breakpoints': [PASS, NO_VARIANTS],  # with custom snapshot and gc-stress.
 
   # TODO(mstarzinger): Optimizing top-level code flushed out some correctness
   # issues on ARM and ARM64.