Plugin Directory: Update API endpoint URLs in unit tests

[SergeyBiryukov]

The tests for locale banner still point to the old plugins-wp URL and currently fail due to a rest_no_route error:

array(3) {
  'code' =>
  string(13) "rest_no_route"
  'message' =>
  string(54) "No route was found matching the URL and request method"
  'data' =>
  array(1) {
    'status' =>
    int(404)
  }
}

...
1) Tests_API_Locale_Banner::test_locale_banner with data set #0 ('en-US,en;q=0.5', array())
Failed asserting that an array has the key 'suggest_string'.

[ocean90]

In 5434:

Plugin Directory: Update API endpoint URLs in unit tests.

Props SergeyBiryukov.
Fixes #2766.