Convert web tests to browser tests for file module

See #2735005: Convert all Simpletest web tests to BrowserTestBase (or UnitTestBase/KernelTestBase) →

Convert

All tests except:

• FileManagedFileElementTest should be split into javascript and BTB test #2809505: Convert AJAX part of \Drupal\file\Tests\FileManagedFileElementTest::testManagedFile to JavascriptTestBase →
• FileFieldWidgetTest should be split into Javascript and BTB test #2809503: Convert AJAX part of \Drupal\file\Tests\FileFieldWidgetTest to JavascriptTestBase →