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/makedevs.tests b/testsuite/makedevs.tests
index 9e068bd..b41614f 100755
--- a/testsuite/makedevs.tests
+++ b/testsuite/makedevs.tests
@@ -2,7 +2,7 @@
 # Copyright 2008 by Denys Vlasenko
 # Licensed under GPL v2, see file LICENSE for details.
 
-. testing.sh
+. ./testing.sh
 
 # ls -ln is showing date. Need to remove that, it's variable
 # sed: (1) "maj, min" -> "maj,min" (2) coalesce spaces