Fix d8-os unit test to be skipped for isolates.

test/mjsunit/d8-os.js

Index: test/mjsunit/d8-os.js
diff --git a/test/mjsunit/d8-os.js b/test/mjsunit/d8-os.js
index d0d68a2d929ee7d263f57ce2a1980958eb917a36..239938cd169d73d3fb847cb7e30d0b767f4a2c9c 100644
--- a/test/mjsunit/d8-os.js
+++ b/test/mjsunit/d8-os.js
@@ -96,7 +96,9 @@ if (this.os && os.system) {
     assertThrows("os.chdir(TEST_DIR + '/dir4');", "chdir dir4 I");
     os.rmdir(TEST_DIR + "/dir4");
     assertThrows("os.chdir(TEST_DIR + '/dir4');", "chdir dir4 II");
-    // Set umask.
+
+    // Set umask.  This changes the umask for the whole process and is
+    // the reason why the test cannot be run multi-threaded.
     var old_umask = os.umask(0777);
     // Create a dir we cannot read.
     os.mkdirp(TEST_DIR + "/dir5");