test-lib: separate individual test better in verbose mode.

When running tests with --verbose it is difficult to see where
one test starts and where it ends because everything is printed
in one big lump.
Fix that by printing one single newline between each test.

Signed-off-by: Martin Waitz <tali@admingilde.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>