changed gqlint to lint the file on disk

- added a cd into the direcotry containing the file in the buffer
  in order to properly check for a config file
- added command_callback tests for graphql

test/command_callback/test_graphql_gqlint_command_callbacks.vader

@@ -0,0 +1,11 @@
+Before:
+  call ale#assert#SetUpLinterTest('graphql', 'gqlint')
+
+After:
+  call ale#assert#TearDownLinterTest()
+
+Execute(The linter should run from the directory of the file in the buffer):
+  AssertLinter 'gqlint',
+  \ ale#path#CdString(expand('%:p:h'))
+  \   . 'gqlint --reporter=simple'
+  \   . ' %t'