Fix tests: Set default_locale to 'en'

This change resolves the following error:
```
rspec ./spec/features/modal_search_spec.rb:25

1) ModalSearchDialog Modal search opens and closes and allows user to search, select and remove items
     Failure/Error: <td><%= l(plan.updated_at.to_date, formats: :short) %></td>

     ActionView::Template::Error:
       Translation missing: en-GB.date.formats.default
```
Because en-GB is not a locale used within DMP Assistant, commit f600ba0 removed the related en-GB files from the app.

Setting the locale to 'en-CA' unfortunately breaks the tests as well. There is an open issue that seeks to resolve this: #931

spec/support/locales.rb

@@ -2,7 +2,7 @@
 
 require 'shoulda/matchers'
 
-AVAILABLE_TEST_LOCALES = %w[en-GB en fr de].freeze
+AVAILABLE_TEST_LOCALES = %w[en fr de].freeze
 
 RSpec.configure do |config|
   config.before(:suite) do