[test] Mark slow test on msan

NOTRY=true
TBR=sergiyb@chromium.org

Change-Id: I200c088d3cf1c3c1a16040b1395b534ccd9d9268
Reviewed-on: https://chromium-review.googlesource.com/765471Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49319}

test/mjsunit/mjsunit.status

@@ -401,6 +401,9 @@
   'compiler/osr-one': [PASS, SLOW],
   'compiler/osr-two': [PASS, SLOW],
   'wasm/grow-memory': [PASS, SLOW],
+
+  # Slow test.
+  'regress/regress-779407': [PASS, SLOW],
 }],  # 'msan == True'
 
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