selftests: Add version file to kselftest installation dir

As titled, adding version file to kselftest installation dir, so the user
of the tarball can know which kernel version the tarball belongs to.

Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Tianyi Cui <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/Makefile

···
 		$(MAKE) -s --no-print-directory OUTPUT=$$BUILD_TARGET COLLECTION=$$TARGET \
 			-C $$TARGET emit_tests >> $(TEST_LIST); \
 	done;
+	@VERSION=$$(git describe HEAD 2>/dev/null); \
+	if [ -n "$$VERSION" ]; then \
+		echo "$$VERSION" > $(INSTALL_PATH)/VERSION; \
+		printf "Version saved to $(INSTALL_PATH)/VERSION\n"; \
+	else \
+		printf "Unable to get version from git describe\n"; \
+	fi
 else
 	$(error Error: set INSTALL_PATH to use install)
 endif