Add a function/macro to skip the whole testcase

[fjarri]

@test_skip only skips one test; it is often useful to skip the whole testcase (like we currently skip output capturing testcases on Windows and Julia 0.6). Some @testcase_skip macro, perhaps?

[fjarri]

It seems that calling @test_skip and return will achieve the desired effect. Since @test_ macros are not implemented via exceptions, a return will be necessary anyway.