Makefile: do not build tests

Tests are broken if no EGL support is available.

Reported-by: Peter Korsgaard <jacmet@uclibc.org>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
diff -durN weston-1.3.0.orig/Makefile.am weston-1.3.0/Makefile.am
--- weston-1.3.0.orig/Makefile.am	2013-08-20 20:15:19.000000000 +0200
+++ weston-1.3.0/Makefile.am	2013-10-14 23:08:12.016392097 +0200
@@ -14,7 +14,6 @@
 	clients					\
 	data					\
 	protocol				\
-	tests					\
 	$(wcap_subdir)				\
 	man