t4018 (funcname patterns): make configuration easier to track

Introduce a "test_config" function to set a configuration variable for use by a single test (automatically unsetting it when the assertion finishes). If this function is used consistently, the configuration used in a test_expect_success block can be read at the beginning of that block instead of requiring reading all the tests that come before. So it becomes a little easier to add new tests or rearrange existing ones without fear of breaking configuration. In particular, the test of alternation in xfuncname patterns also checks that xfuncname takes precedence over funcname variable as a sort of side-effect, since the latter leaks in from previous tests. In the new syntax, the test has to say explicitly what variables it is using, making the test clearer and a future regression in coverage from carelessly editing the script less likely. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>