[test] Mark slow tests on msan.

TBR=svenpanne@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1159743002

Cr-Commit-Position: refs/heads/master@{#28611}

test/cctest/cctest.status

@@ -194,6 +194,12 @@
 ['msan == True', {
   # ICU upstream issues.
   'test-strings/CountBreakIterator': [SKIP],
+
+  # Slow tests.
+  'test-api/Threading1': [PASS, SLOW],
+  'test-api/Threading2': [PASS, SLOW],
+  'test-api/Threading3': [PASS, SLOW],
+  'test-api/Threading4': [PASS, SLOW],
 }],  # 'msan == True'
 
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