Make it easier to run tests locally

Certain tests could break if you ran them separately from other tests.
They have been patched.

`run-tests` now has a `--fast` option which runs tests with only the
fastest Vim version ALE tests with, and the custom checks.

test/fixers/test_prettier_eslint_fixer.callback.vader

@@ -1,5 +1,8 @@
 Before:
   call ale#assert#SetUpFixerTest('javascript', 'prettier_eslint')
+  Save g:ale_command_wrapper
+
+  let g:ale_command_wrapper = ''
 
 After:
   call ale#assert#TearDownFixerTest()