test: Set VIMRUNTIME before running tests

This is required for testing scripts under the 'runtime' directory

cmake/RunTests.cmake

@@ -1,6 +1,8 @@
 get_filename_component(BUSTED_DIR ${BUSTED_PRG} PATH)
 set(ENV{PATH} "${BUSTED_DIR}:$ENV{PATH}")
 
+set(ENV{VIMRUNTIME} ${WORKING_DIR}/runtime)
+
 if(NVIM_PRG)
   set(ENV{NVIM_PROG} "${NVIM_PRG}")
 endif()