tests: use a lowercase "usage:" string

Adjust test commands and test suites so that their
usage strings are consistent with Git.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-genrandom.c

@@ -12,7 +12,7 @@ int main(int argc, char *argv[])
 	unsigned char *c;
 
 	if (argc < 2 || argc > 3) {
-		fprintf(stderr, "Usage: %s <seed_string> [<size>]\n", argv[0]);
+		fprintf(stderr, "usage: %s <seed_string> [<size>]\n", argv[0]);
 		return 1;
 	}