Add fixer for Go modules (#1873)

* Add fixer for Go modules

test/command_callback/test_gobuild_command_callback.vader

@@ -1,9 +1,12 @@
 Before:
+  Save g:ale_go_go_executable
+
   call ale#assert#SetUpLinterTest('go', 'gobuild')
 
   WithChainResults ['/foo/bar', '/foo/baz']
 
 After:
+  Restore
   call ale#assert#TearDownLinterTest()
 
 Execute(The default commands should be correct):
@@ -17,3 +20,12 @@ Execute(Extra options should be supported):
   AssertLinter 'go',
   \ 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
   \   . 'go test --foo-bar -c -o /dev/null ./'
+
+  let g:ale_go_gobuild_options = ''
+
+Execute(The executable should be configurable):
+  let g:ale_go_go_executable = 'foobar'
+
+  AssertLinter 'foobar',
+  \ 'cd ' . ale#Escape(expand('%:p:h')) . ' && '
+  \   . 'foobar test -c -o /dev/null ./'