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/tail.tests b/testsuite/tail.tests
index d97bf7b..d2427d4 100755
--- a/testsuite/tail.tests
+++ b/testsuite/tail.tests
@@ -3,7 +3,7 @@
 # Copyright 2009 by Denys Vlasenko <vda.linux@googlemail.com>
 # Licensed under GPL v2, see file LICENSE for details.
 
-. testing.sh
+. ./testing.sh
 
 # testing "test name" "command" "expected result" "file input" "stdin"
 #   file input will be file called "input"