Set prettier working directory to where .prettierignore is (#3101)

Prettier does not use `.prettierignore` unless the current directory is the root where the `.prettierignore` file resides.

* Update Prettier tests
* Look for prettierignore to determine project root

test/fixers/test_prettier_fixer_callback.vader

@@ -297,6 +297,17 @@ Execute(Should set --parser for experimental language, Handlebars):
   \     . ' --stdin-filepath %s --stdin',
   \ }
 
+Execute(Changes to directory where .prettierignore is found):
+  call ale#test#SetFilename('../prettier-test-files/with_prettierignore/src/testfile.js')
+
+  GivenCommandOutput ['1.6.0']
+  AssertFixer
+  \ {
+  \   'command': ale#path#CdString(expand('%:p:h:h'))
+  \     . ale#Escape(g:ale_javascript_prettier_executable)
+  \     . ' --stdin-filepath %s --stdin',
+  \ }
+
 Execute(The prettier_d post-processor should permit regular JavaScript content):
   AssertEqual
   \ [