t0014: introduce an alias testing suite

t0014: introduce an alias testing suite

Introduce a testing suite that is dedicated to aliases.
For now, check only if nested aliases work and if looping
aliases are detected successfully.

The looping aliases check for mixed execution is there but
disabled, because it is blocking the test suite for a full
minute. As soon as there is a solution for loops using
external commands, it should be enabled.

Signed-off-by: Tim Schumacher <timschumi@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>