Close #2281 - Separate cwd commands from commands

Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.

test/fixers/test_black_fixer_callback.vader

@@ -1,30 +1,23 @@
 Before:
-  Save g:ale_python_black_executable
-  Save g:ale_python_black_options
+  call ale#assert#SetUpFixerTest('python', 'black')
 
-  " Use an invalid global executable, so we don't match it.
-  let g:ale_python_black_executable = 'xxxinvalid'
-  let g:ale_python_black_options = ''
-  let g:ale_python_black_auto_pipenv = 0
-
-  call ale#test#SetDirectory('/testplugin/test/fixers')
   silent cd ..
   silent cd command_callback
   let g:dir = getcwd()
-
   let b:bin_dir = has('win32') ? 'Scripts' : 'bin'
 
 After:
-  Restore
+  call ale#assert#TearDownFixerTest()
 
+  unlet! g:dir
   unlet! b:bin_dir
 
-  call ale#test#RestoreDirectory()
-
 Execute(The black callback should return the correct default values):
   silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py')
   AssertEqual
-  \ {'command': ale#path#BufferCdString(bufnr('')) . ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/black')) . ' -'},
+  \ {
+  \   'cwd': '%s:h',
+  \   'command': ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/black')) . ' -'},
   \ ale#fixers#black#Fix(bufnr(''))
 
 Execute(The black callback should include options):
@@ -40,6 +33,7 @@ Execute(The black callback should include --pyi for .pyi files):
   let g:ale_python_black_change_directory = 0
 
   silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.pyi')
+
   AssertEqual
   \ {'command': ale#Escape(ale#path#Simplify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/black')) . ' --pyi -' },
   \ ale#fixers#black#Fix(bufnr(''))
@@ -48,7 +42,7 @@ Execute(Pipenv is detected when python_black_auto_pipenv is set):
   let g:ale_python_black_auto_pipenv = 1
   let g:ale_python_black_change_directory = 0
 
-  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+  call ale#test#SetFilename('../python_fixtures/pipenv/whatever.py')
 
   AssertEqual
   \ {'command': ale#Escape('pipenv') . ' run black -'},