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/command_callback/test_elixir_credo_command_callback.vader

@@ -0,0 +1,43 @@
+Before:
+  call ale#assert#SetUpLinterTest('elixir', 'credo')
+  call ale#test#SetFilename('elixir_paths/mix_project/lib/app.ex')
+
+
+After:
+  unlet! g:ale_elixir_credo_strict
+
+  call ale#assert#TearDownLinterTest()
+
+Execute(Builds credo command with normal project):
+  AssertLinterCwd ale#path#Simplify(g:dir . '/elixir_paths/mix_project')
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo suggest --format=flycheck --read-from-stdin %s'
+
+Execute(Builds credo command with umbrella project):
+  call ale#test#SetFilename('elixir_paths/umbrella_project/apps/mix_project/lib/app.ex')
+
+  AssertLinterCwd ale#path#Simplify(g:dir . '/elixir_paths/umbrella_project')
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo suggest --format=flycheck --read-from-stdin %s'
+
+Execute(Builds credo command with --strict mode when set to 1):
+  let g:ale_elixir_credo_strict = 1
+
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo --strict --format=flycheck --read-from-stdin %s'
+
+Execute(Builds credo command with suggest mode by default):
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo suggest --format=flycheck --read-from-stdin %s'
+
+Execute(Builds credo command with suggest mode when set to 0):
+  let g:ale_elixir_credo_strict = 0
+
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo suggest --format=flycheck --read-from-stdin %s'
+
+Execute(Builds credo command with a custom config file):
+  let g:ale_elixir_credo_config_file = '/home/user/custom_credo.exs'
+
+  AssertLinter 'mix',
+  \ 'mix help credo && mix credo suggest --config-file /home/user/custom_credo.exs --format=flycheck --read-from-stdin %s'