t: prepare conditional test execution for `set -e`

t: prepare conditional test execution for `set -e`

We have some test in our test suite where we use the pattern of
`test ... && test_expect_succeess` to conditionally execute a test. The
problem is that when we decide to not execute the test, we'll indeed
skip the test, but the overall statement will also be unsuccessful. This
will become a problem once we enable `set -e`.

Prepare for this future by turning this into a proper conditional, which
is also a bit easier to read overall.

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