travis-ci: run previously failed tests first, then slowest to fastest

travis-ci: run previously failed tests first, then slowest to fastest

The Travis-CI machines are in a clean state in the beginning of every run
(transient by default). Use the Travis-CI cache feature to make the prove
state persistent across consecutive Travis-CI runs on the same branch.
This allows to run previously failed tests first and run remaining tests
in slowest to fastest order. As a result it is less likely that Travis-CI
needs to wait for a single test at the end which speeds up the test suite
execution by ~2 min.

Travis-CI can only cache entire directories. Prove stores the .prove file
always in the t/ directory but we don't want to cache the entire t/ directory.
Therefore we create a symlink from $HOME/travis-cache/.prove to t/.prove and
cache the $HOME/travis-cache directory.

Unfortunately the cache feature is only available (for free) on the
Travis-CI Linux environment.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>