Improve performance when using gometalinter (#566)

* Improve performance when using gometalinter

Before this change when I opened a big project that had 6000+ warnings/errors it took ages to get the actual warnings/errors and it caused my CPU to be busy for quite some time. The call to gometalinter alone took about 24 seconds, but after that vim was struggling as well.

After this change the gometalinter call just takes 2 seconds and nothing noticable happens with the CPU and/or vim.

* Removed obsolete test

This logic is no longer done by the `ale` plugin, but by `gometalinter` itself.

ale_linters/go/gometalinter.vim

@@ -6,7 +6,7 @@ if !exists('g:ale_go_gometalinter_options')
 endif
 
 function! ale_linters#go#gometalinter#GetCommand(buffer) abort
-    return 'gometalinter '
+    return 'gometalinter --include=''^' . expand('%:p') . '.*$'' '
     \   . ale#Var(a:buffer, 'go_gometalinter_options')
     \   . ' ' . ale#Escape(fnamemodify(bufname(a:buffer), ':p:h'))
 endfunction
@@ -21,11 +21,6 @@ function! ale_linters#go#gometalinter#Handler(buffer, lines) abort
     let l:output = []
 
     for l:match in ale_linters#go#gometalinter#GetMatches(a:lines)
-        " Omit errors from files other than the one currently open
-        if !ale#path#IsBufferPath(a:buffer, l:match[1])
-            continue
-        endif
-
         call add(l:output, {
         \   'lnum': l:match[2] + 0,
         \   'col': l:match[3] + 0,