Remove Unused local variable $base_url from /core/modules/locale/lib/Drupal/locale/Tests/LocaleTranslationUiTest.php

Part of meta-issue #2002650: [meta, no patch] improve maintainability by removing unused local variables

File /core/modules/locale/lib/Drupal/locale/Tests/LocaleTranslationUiTest.php

Line 49: Unused local variable $base_url
Line 61: Unused local variable $language_indicator
Line 97: Unused local variable $edit
Line 279: Unused local variable $base_url
Line 319: Unused local variable $key
Line 335: Unused local variable $base_url
Line 348: Unused local variable $language_indicator
Line 513: Unused local variable $source
Line 531: Unused local variable $source