test-lib: add test_config_global variant

The point of test_config is to simultaneously set a config variable and register its cleanup handler, like: test_config core.foo bar However, it stupidly assumes that $1 contained the name of the variable, which means it won't work for: test_config --global core.foo bar We could try to parse the command-line ourselves and figure out which parts need to be fed to test_unconfig. But since this is likely the most common variant, it's much simpler and less error-prone to simply add a new function. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>