t5100-mailinfo: replace common path prefix with variable

t5100-mailinfo: replace common path prefix with variable

Many tests need to store data in a file, and repeat the same pattern to
refer to that path:

    "$TEST_DIRECTORY"/t5100/

Create a variable that contains this path, and use that instead.

While we're making this change, make sure the quotes are not just around
the variable, but around the entire string to not give the impression
we want shell splitting to affect the other variables.

Signed-off-by: Kevin Daudt <me@ikke.info>
Signed-off-by: Junio C Hamano <gitster@pobox.com>