OK, that explains the failure. You must have set the OS X crash reporter default to "Developer" mode on that machine at some point.  In that case, code that is now in the SuppressCrashReport context manager in Lib/test/support/__init__.py checks for that setting by "shelling out" on OS X to /usr/bin/defaults for each use of the context manager in tests and, if set to "Developer", outputs that message to stdout.  So that will interfere with a test like this where the contents of stdout is used as part of the test.  But the code seems problematic in a couple of other respects.  You can still get some crash popups even if the preference is not set to Developer.  Also if the preference has never been set, you get a spurious error message to stderr for each test case that uses the context manager:

2015-05-14 14:34:44.185 defaults[90018:2205666]
The domain/default pair of (/Users/nad/Library/Preferences/com.apple.CrashReporter, DialogType) does not exist

My initial reaction without full testing of the effects of the Crash Reporter settings would be to: (1) not print a message to stdout since none of the other non-OS X cases do; (2) cache the results of the initial /usr/bin/defaults call.  As a workaround, you could comment out the print.

https://developer.apple.com/library/mac/technotes/tn2004/tn2123.html