Skip thread-hooks related tests on 10.6 or later

remove tests requires "thread-hooks" in tc_threads on 10.6 or later.
let to pass all tests without "test-unit2" library.

git-svn-id: http://rubycocoa.svn.sourceforge.net/svnroot/rubycocoa/trunk/src@2415 f4f36d40-9719-0410-9868-dde021a23eda