t: prepare execution of potentially failing commands for `set -e`

t: prepare execution of potentially failing commands for `set -e`

Several of our tests verify whether a certain binary can be executed,
potentially skipping tests in case we cannot, for example because the
binary doesn't exist. In those cases we often run the binary outside of
any conditionally.

This will start to fail once we enable `set -e`, as that will cause us
to bail out the test immediately. Improve these tests by executing them
inside of a conditional instead.

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