[mjsunit] Disable test unsuitable for GC stress

regress-6989 uses assertOptimized and will hence fail randomly depending
on GC timing. This means we should not run it under GC stress.

This can be verified by running the test under GC stress and changing the
--gc-interval. It is easy to find values for which the test fails, and
values for which the test passes.

Change-Id: Ibd8bfbc1712ad60830255e4d89ea795023134891
Reviewed-on: https://chromium-review.googlesource.com/c/1396078Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58540}

test/mjsunit/mjsunit.status

@@ -268,6 +268,7 @@
   'regress/regress-trap-allocation-memento': [SKIP],
   'regress/regress-2249': [SKIP],
   'regress/regress-4121': [SKIP],
+  'regress/regress-6989': [SKIP],
   'compare-known-objects-slow': [SKIP],
   'compiler/array-multiple-receiver-maps': [SKIP],
   # Tests taking too long