tests: Improve naming of build common test variables

Use the mbedtls_test_ prefix for (c)make variables
related to test common code.

This aligns with the prefix used for the common test
functions.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/tests/Makefile b/tests/Makefile
index 6f3179c..68f85d2 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -77,7 +77,7 @@
 $(MBEDLIBS):
 	$(MAKE) -C ../library
 
-MBEDTESTS_OBJS=$(patsubst %.c,%.o,$(wildcard src/*.c))
+MBEDTLS_TEST_OBJS=$(patsubst %.c,%.o,$(wildcard src/*.c))
 
 # Rule to compile common test C files in src folder
 src/%.o : src/%.c
@@ -112,9 +112,9 @@
 		-o .
 
 
-$(BINARIES): %$(EXEXT): %.c $(MBEDLIBS) $(MBEDTESTS_OBJS)
+$(BINARIES): %$(EXEXT): %.c $(MBEDLIBS) $(MBEDTLS_TEST_OBJS)
 	echo "  CC    $<"
-	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $< $(MBEDTESTS_OBJS) $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
+	$(CC) $(LOCAL_CFLAGS) $(CFLAGS) $< $(MBEDTLS_TEST_OBJS) $(LOCAL_LDFLAGS) $(LDFLAGS) -o $@
 
 # Some test suites require additional header files.
 $(filter test_suite_psa_crypto%, $(BINARIES)): include/test/psa_crypto_helpers.h