test-lib: introduce required prereq for test runs

test-lib: introduce required prereq for test runs

In certain environments or for specific test scenarios we might expect a
specific prerequisite check to succeed. Therefore we would like to abort
running our tests if this is not the case.

To remedy this we add the environment variable GIT_TEST_REQUIRE_PREREQ
which can be set to a space separated list of prereqs. If one of these
prereq tests fail then the whole test run will abort.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>