Clean up the mix format command and tests

autoload/ale/fixers/mix_format.vim

@@ -12,7 +12,9 @@ function! ale#fixers#mix_format#GetCommand(buffer) abort
     let l:executable = ale#Escape(ale#fixers#mix_format#GetExecutable(a:buffer))
     let l:options = ale#Var(a:buffer, 'elixir_mix_format_options')
 
-    return l:executable . ' format ' . l:options . ' %t'
+    return l:executable . ' format'
+    \   . (!empty(l:options) ? ' ' . l:options : '')
+    \   . ' %t'
 endfunction
 
 function! ale#fixers#mix_format#Fix(buffer) abort

test/fixers/test_mix_format_fixer_callback.vader

@@ -1,11 +1,15 @@
 Before:
-  call ale#test#SetDirectory('/testplugin/test/fixers')
   Save g:ale_elixir_mix_executable
   Save g:ale_elixir_mix_format_options
 
   let g:ale_elixir_mix_executable = 'xxxinvalid'
+  let g:ale_elixir_mix_format_options = ''
+
+  call ale#test#SetDirectory('/testplugin/test/fixers')
 
 After:
+  Restore
+
   call ale#test#RestoreDirectory()
 
 Execute(The mix_format callback should return the correct default values):
@@ -15,7 +19,7 @@ Execute(The mix_format callback should return the correct default values):
   \ {
   \   'read_temporary_file': 1,
   \   'command': ale#Escape('xxxinvalid')
-  \     . ' format  %t',
+  \     . ' format %t',
   \ },
   \ ale#fixers#mix_format#Fix(bufnr(''))