[test] Mark tests slow with isolates testing

The isolates step times out due to some very slow tests. Marking as slow
changes the test order and increases throughput.

NOTRY=true
TBR=yangguo@chromium.org

Change-Id: Iaaf6fe93e7f0e17266923d1ab6f0fe7b09abea9e
Reviewed-on: https://chromium-review.googlesource.com/1213212Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55734}

test/mjsunit/mjsunit.status

@@ -654,6 +654,15 @@
   'big-object-literal': [SKIP],
 }],  # 'system == macos'
 
+##############################################################################
+['isolates', {
+  # Slow tests.
+  'es6/typedarray-of': [PASS, SLOW],
+  'regress/regress-crbug-854299': [PASS, SLOW],
+  'regress/wasm/regress-810973': [PASS, SLOW],
+  'wasm/compare-exchange-stress': [PASS, SLOW],
+}],  # 'isolates'
+
 ##############################################################################
 ['deopt_fuzzer == True', {