For some reason, the newly-added Tk instantiation check causes the OS X Cocoa Tk to segfault when tests are run under regrtest -jn option, which causes each test to be run under a separate subprocess called from a separate thread.  Somewhat surprisingly, the segfault doesn't seem to happen under the same conditions with 2.7, only with 3.4 and default, so there is something more at play here.  But with the imminent tagging of 3.4.1rc1, I think it is better to disable the check for 3.4 (and default) on OS X pending further investigation.  The check is redundant: if Tk doesn't work for some reason, the individual test should fail anyway, just a bit later.