app/tests: Rename test-layers to test-core

Rename test-layers to test-core so we can put more tests there, like
color processing algorithm tests for example.

app/tests/.gitignore

@@ -4,8 +4,8 @@
 Makefile
 Makefile.in
 libgimpapptestutils.a
+test-core*
 test-layer-grouping*
-test-layers*
 test-session-management*
 test-ui*
 test-window-management*

app/tests/Makefile.am

@@ -11,7 +11,7 @@ TESTS_ENVIRONMENT = \
 	GIMP_TESTING_ABS_TOP_BUILDDIR=@abs_top_builddir@
 
 TESTS = \
-	test-layers		\
+	test-core		\
 	test-session-management	\
 	test-ui			\
 	test-xcf

app/tests/test-core.c

@@ -31,7 +31,7 @@
 #define GIMP_TEST_IMAGE_SIZE 100
 
 #define ADD_TEST(function) \
-  g_test_add ("/gimp-layers/" #function, \
+  g_test_add ("/gimp-core/" #function, \
               GimpTestFixture, \
               gimp, \
               gimp_test_image_setup, \