t/test-lib.sh: provide a shell implementation of the 'yes' utility

t/test-lib.sh: provide a shell implementation of the 'yes' utility

Some platforms (IRIX 6.5, Solaris 7) do not provide the 'yes' utility.
Currently, some tests, including t7610 and t9001, try to call this program.
Due to the way the tests are structured, the tests still pass even though
this program is missing.  Rather than succeeding by chance, let's provide
an implementation of the simple 'yes' utility in shell for all platforms to
use.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>