tests: add CGIT_TEST_OPTS variable to Makefile

This allows running the entire test suite with a set of command-line
options.  For example:

	make test CGIT_TEST_OPTS=--valgrind

Signed-off-by: John Keeping <john@keeping.me.uk>

tests/Makefile

@@ -5,7 +5,7 @@ T = $(wildcard t[0-9][0-9][0-9][0-9]-*.sh)
 all: $(T)
 
 $(T):
-	@./$@
+	@./$@ $(CGIT_TEST_OPTS)
 
 clean:
 	$(RM) -rf trash