Build: Add --disable-tests configure flag to avoid tests build

Build: Add --disable-tests configure flag to avoid tests build

Tests are still being built by default. However this request
came from OSVs in order to allow them to include i-g-t in their
distributions by default avoiding adding more and more dependencies
since we are improving and adding more and more tests.

v2: wait for Ben's spacing fixes and adjusted for new space rules.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Acked-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>