test-lib: Document short options in t/README

Most arguments that could be provided to a test have short forms.
Unless documented, the only way to learn them is to read the code.

Signed-off-by: Ilya Bobyr <ilya.bobyr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
test-lib: tests skipped by GIT_SKIP_TESTS say so

We used to show "(missing )" next to tests skipped because they are
specified in GIT_SKIP_TESTS.  Use "(GIT_SKIP_TESTS)" instead.

Plus tests that check basic GIT_SKIP_TESTS functions.

Signed-off-by: Ilya Bobyr <ilya.bobyr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
test-lib: '--run' to run only specific tests

Allow better control of the set of tests that will be executed for a
single test suite.  Mostly useful while debugging or developing as it
allows to focus on a specific test.

Signed-off-by: Ilya Bobyr <ilya.bobyr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>