Set up test environment properly

To make the G_TEST_SRCDIR and G_TEST_BUILDDIR variables
reach our test binaries, we have to add it to the
TESTS_ENVIRONMENT variable.
diff --git a/glib-tap.mk b/glib-tap.mk
index 360ef62..6b9d107 100644
--- a/glib-tap.mk
+++ b/glib-tap.mk
@@ -1,5 +1,11 @@
 # GLIB - Library of useful C routines
 
+TESTS_ENVIRONMENT= \
+	G_TEST_SRCDIR="$(abs_srcdir)" 		\
+	G_TEST_BUILDDIR="$(abs_builddir)" 	\
+	G_DEBUG=gc-friendly 			\
+	MALLOC_CHECK_=2 			\
+	MALLOC_PERTURB_=$$(($${RANDOM:-256} % 256))
 TEST_LOG_DRIVER = env AM_TAP_AWK='$(AWK)' $(SHELL) $(top_srcdir)/tap-driver.sh
 LOG_COMPILER = $(top_srcdir)/tap-test