t0303: resurrect commit message as test documentation

t0303: resurrect commit message as test documentation

The commit message which added those tests (861444f 't: add test
harness for external credential helpers' 2011-12-10) provided nice
documentation in the commit message. Let's make it more visible
by putting it in the test description.

The documentation is updated to reflect the fact that
GIT_TEST_CREDENTIAL_HELPER must be set for
GIT_TEST_CREDENTIAL_HELPER_TIMEOUT to be used
and GIT_TEST_CREDENTIAL_HELPER_SETUP can be used.

Based-on-commit-message-by: Jeff King <peff@peff.net>
Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>