t: prepare `test_must_fail ()` for `set -e`

The helper function `test_must_fail ()` executes a specific Git command that may or may not fail in a specific way. This is done by executing the command in question and then comparing its exit code against a set of conditions. This works, but once we run our test suite with `set -e` we may bail out of `test_must_fail ()` early in case the command actually fails, even though we expect it to fail. Prepare for this change by handling the failed case with `||`. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>