[test] Skip slow tests in TSAN

This skips two tests in TSAN with stress mode only. The tests are
particularly slow with isolates testing, but they're also the two
lowest hanging fruits in normal TSAN testing.

TBR=sergiyb@chromium.org
NOTRY=true

Bug: v8:8009
Change-Id: Ic262fc39dee8ee0d8d1fdad10beced0e8f9c87a0
Reviewed-on: https://chromium-review.googlesource.com/1160860
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54881}

test/mjsunit/mjsunit.status

@@ -773,6 +773,10 @@
 
   # Flaky crash on Odroid devices: https://crbug.com/v8/7678
   'regress/regress-336820': [PASS, ['arch == arm and not simulator_run', SKIP]],
+
+  # Too slow for TSAN in stress mode.
+  'es6/classes': [PASS, ['tsan', SKIP]],
+  'regress/regress-1122': [PASS, ['tsan', SKIP]],
 }],  # variant == stress
 
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