t: rename t2014-switch.sh to t2014-checkout-switch.sh

The old name does not really say that this is about 'checkout -b'. See
49d833dc (Revert "checkout branch: prime cache-tree fully" -
2009-05-12) for more information
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>