drm/vkms: Add KUnit test scaffolding

Add the required boilerplate to start creating KUnit test.

To run the tests:

$ ./tools/testing/kunit/kunit.py run \
--kunitconfig=drivers/gpu/drm/vkms/tests

Reviewed-by: Louis Chauvet <[e-mail]>
Co-developed-by: Arthur Grillo <[e-mail]>
Signed-off-by: Arthur Grillo <[e-mail]>
Co-developed-by: Louis Chauvet <[e-mail]>
Signed-off-by: Louis Chauvet <[e-mail]>
Signed-off-by: José Expósito <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Maxime Ripard <[e-mail]>

drivers/gpu/drm/vkms/Kconfig

···
 	  a VKMS.
 
 	  If M is selected the module will be called vkms.
+
+config DRM_VKMS_KUNIT_TEST
+	tristate "KUnit tests for VKMS" if !KUNIT_ALL_TESTS
+	depends on DRM_VKMS && KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  This builds unit tests for VKMS. This option is not useful for
+	  distributions or general kernels, but only for kernel
+	  developers working on VKMS.
+
+	  For more information on KUnit and unit tests in general,
+	  please refer to the KUnit documentation in
+	  Documentation/dev-tools/kunit/.
+
+	  If in doubt, say "N".

drivers/gpu/drm/vkms/Makefile

···
 	vkms_connector.o
 
 obj-$(CONFIG_DRM_VKMS) += vkms.o
+obj-$(CONFIG_DRM_VKMS_KUNIT_TEST) += tests/

drivers/gpu/drm/vkms/tests/.kunitconfig

···
+CONFIG_KUNIT=y
+CONFIG_DRM=y
+CONFIG_DRM_VKMS=y
+CONFIG_DRM_VKMS_KUNIT_TEST=y

drivers/gpu/drm/vkms/tests/Makefile

···
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_DRM_VKMS_KUNIT_TEST) += vkms_config_test.o

drivers/gpu/drm/vkms/tests/vkms_config_test.c

···
+// SPDX-License-Identifier: GPL-2.0+
+
+#include <kunit/test.h>
+
+MODULE_IMPORT_NS("EXPORTED_FOR_KUNIT_TESTING");
+
+static struct kunit_case vkms_config_test_cases[] = {
+	{}
+};
+
+static struct kunit_suite vkms_config_test_suite = {
+	.name = "vkms-config",
+	.test_cases = vkms_config_test_cases,
+};
+
+kunit_test_suite(vkms_config_test_suite);
+
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Kunit test for vkms config utility");