test: disable librt shmtest on non-MMU systems

This test fails for non-MMU systems, because it uses fork()
Thanks to Waldemar Brodkorb!

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

test/librt/Makefile.in

@@ -2,3 +2,7 @@
 # Licensed under the LGPL v2.1, see the file COPYING.LIB in this tarball.
 
 LDFLAGS_shmtest := -lrt
+
+ifeq ($(ARCH_USE_MMU),)
+TESTS_DISABLED := shmtest
+endif