fsmonitor: update GIT_TEST_FSMONITOR support

fsmonitor: update GIT_TEST_FSMONITOR support

Rename GIT_FSMONITOR_TEST to GIT_TEST_FSMONITOR for consistency with the
other GIT_TEST_ special setups and properly document its use.

Add logic in t/test-lib.sh to give a warning when the old variable is set to
let people know they need to update their environment to use the new
variable.

Remove the outdated instructions on how to run the test suite utilizing
fsmonitor now that it is properly documented in t/README.

Signed-off-by: Ben Peart <Ben.Peart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>