t7527: fix broken TAP output

t7527: fix broken TAP output

Before running the tests in t7527 we first verify whether the fsmonitor
even works, which seems to depend on the actual filesystem that is in
use. The verification executes outside of any prerequisite or test body,
so its stdout/stderr is not being redirected.

The consequence of this is that any command that prints to stdout/stderr
may break the TAP specification by printing invalid lines. And in fact
we already do that, as git-init(1) prints the path to the created Git
repository by default.

Fix this issue by moving the logic into a lazy prerequisite.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>