[mac] Test_org_eclipse_swt_browser_Browser.test_execute_and_closeListener is flaky

[fedejeanne]

Failed for Mac in this build of #1603

Error:  org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser.test_execute_and_closeListener[browser flags: 0] -- Time elapsed: 18.70 s <<< FAILURE!
java.lang.AssertionError: Either browser.execute() did not work (if you still see the html page) or closeListener Was not triggered if browser looks disposed, but test still fails.
	at org.junit.Assert.fail(Assert.java:89)
	at org.junit.Assert.assertTrue(Assert.java:42)
	at org.eclipse.swt.tests.junit.Test_org_eclipse_swt_browser_Browser.test_execute_and_closeListener(Test_org_eclipse_swt_browser_Browser.java:1805)
...