t5531: fix test description

t5531: fix test description

The description of the test was not enclosed in single quotes, which
broke the coloring scheme that I am used to.  Upon closer inspection
the test is good, but the description is a bit vague. So extend the
description of the first test.

While at it align the description of the file to match what we actually
test in the file.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>