test-lib: make BAIL_OUT() work in tests and prereq

test-lib: make BAIL_OUT() work in tests and prereq

BAIL_OUT() is meant to abort the whole test run and print a message with
a standard prefix that can be parsed to stdout. Since for every test the
normal fd`s are redirected in test_eval_ this output would not be seen
when used within the context of a test or prereq like we do in
test_have_prereq(). To make this function work in these contexts we move
the setup of the fd aliases a few lines up before the first use of
BAIL_OUT() and then have this function always print to the alias.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>