Fix syntax tests fail cause of bundled markdown

We prepend the paths to runtimepath so that vim bundled markdown
syntax is not loaded.

Author: shirosaki

test/vimrc

@@ -3,11 +3,11 @@ set nocompatible
 let $LANG='en_US'
 
 filetype off
-set runtimepath+=../
-set runtimepath+=../build/tabular/
-set runtimepath+=../build/vim-toml/
-set runtimepath+=../build/vim-json/
-set runtimepath+=../build/vader.vim/
+execute 'set runtimepath=../,' . &runtimepath
+execute 'set runtimepath=../build/tabular/,' . &runtimepath
+execute 'set runtimepath=../build/vim-toml/,' . &runtimepath
+execute 'set runtimepath=../build/vim-json/,' . &runtimepath
+execute 'set runtimepath=../build/vader.vim/,' . &runtimepath
 filetype on
 
 filetype plugin indent on