The 'skip_all' facility cannot be used after one or more tests
have been executed using (for example) 'test_expect_success'.
To do so results in invalid TAP output, which leads to 'prove'
complaining of "Parse errors: No plan found in TAP output".

Add a check for such invalid usage and abort the test with an
error message to alert the test author.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>