build: Use AM_TESTS_ENVIRONMENT rather than TESTS_ENVIRONMENT

TESTS_ENVIRONMENT is reserved for the user to be able to set when
running the tests. AM_TESTS_ENVIRONMENT is for the tests’ Makefile to
set itself.

https://www.gnu.org/software/automake/manual/html_node/Scripts_002dbased-Testsuites.html

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #880
Approved by: cgwalters

buildutil/glib-tap.mk

@@ -1,6 +1,6 @@
 # GLIB - Library of useful C routines
 
-TESTS_ENVIRONMENT= \
+AM_TESTS_ENVIRONMENT= \
 	G_TEST_SRCDIR="$(abs_srcdir)" 		\
 	G_TEST_BUILDDIR="$(abs_builddir)" 	\
 	UNINSTALLEDTESTS=1                      \