CMake: Rename target "test" to "functionaltest".

Remove build warning:

    The target name "test" is reserved or not valid for certain CMake
    features, such as generator expressions, and may result in undefined
    behavior.

CMakeLists.txt

@@ -249,7 +249,7 @@ if(BUSTED_PRG)
       -P ${CMAKE_MODULE_PATH}/RunTests.cmake
     DEPENDS nvim-test unittest-headers)
 
-  add_custom_target(test
+  add_custom_target(functionaltest
     COMMAND ${CMAKE_COMMAND}
       -DBUSTED_PRG=${BUSTED_PRG}
       -DWORKING_DIR=${CMAKE_CURRENT_SOURCE_DIR}

Makefile

@@ -77,8 +77,10 @@ endif
 oldtest: | nvim
 	+$(SINGLE_MAKE) -C src/nvim/testdir $(MAKEOVERRIDES)
 
-test: | nvim
-	+$(BUILD_CMD) -C build test
+functionaltest: | nvim
+	+$(BUILD_CMD) -C build functionaltest
+
+test: functionaltest
 
 unittest: | nvim
 	+$(BUILD_CMD) -C build unittest
@@ -93,4 +95,4 @@ distclean: clean
 install: | nvim
 	+$(BUILD_CMD) -C build install
 
-.PHONY: test unittest clean distclean nvim cmake deps install
+.PHONY: test functionaltest unittest clean distclean nvim cmake deps install