t2106: ensure that the checkout fails for the expected reason

t2106: ensure that the checkout fails for the expected reason

During the transition of the test suite to a new default branch name, it
was noticed that this test case succeeded for the wrong reason when the
default branch name was overridden.

While we fixed that in the previous commit, let's make sure that we look
for a tell-tale in the error message that the `git checkout` failed for
the reason we wanted it to fail.

Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Junio C Hamano <redacted>