t5608: avoid say() and use "skip_all" instead for consistency

t5608: avoid say() and use "skip_all" instead for consistency

Printing a message directly to stdout could affect TAP processing
and is not really needed, as there is a standard way to skip all
tests that could be used instead, while printing an equivalent
message.

While at it; update the message to better reflect that since
a85efb5985 (t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool,
2019-11-22), the enabling variable should be a recognized boolean
(ex: true, false, 1, 0) and get rid of the prerequisite that used
to guard all the tests, since "skip_all" is just much faster and
idempotent.

Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>