Fix running tests for OS X framework targets

The OS X logic test query and runner do exactly what we need to do for a framework target, so I figure just detect the odd case of a framework as `TEST_HOST` and call that a logic test.

I'm not particularly happy about `TestHostIsFrameworkExecutable` loading the framework bundle to carry out its tests; I'm worried about categories inadvertently overriding methods or other possible weirdness. But relying on the path alone is naive, as frameworks can include executables (perhaps as resources) that are legitimate test hosts. Suggestions welcome.

Fixes #251.