tests: fix POSIX shell source style

The source command does not search $PWD, and bash recently fixed itself to
follow this behavior for /bin/sh.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/testsuite/patch.tests b/testsuite/patch.tests
index 1bdc0c2..f3e6071 100755
--- a/testsuite/patch.tests
+++ b/testsuite/patch.tests
@@ -2,7 +2,7 @@
 # Copyright 2008 by Denys Vlasenko
 # Licensed under GPL v2, see file LICENSE for details.
 
-. testing.sh
+. ./testing.sh
 
 # testing "test name" "options" "expected result" "file input" "stdin"