Change Namespace for RestUITest.php tests

Problem/Motivation

WebDriverTestBase is a FunctionalJavascript test.

The namespace of tests should be \Drupal\Tests\$extension\$suite_type and tests must be within a tests/src/$suite_type directory.

PHPUnit file structure, namespace, and required metadata

PHPUnit-based: Generally, the other suites must be within a tests/src/$suite_type directory. These tests will have a namespace of \Drupal\Tests\$extension\$suite_type. Note that there are exceptions to this naming scheme, illustrated below.

Proposed resolution

Change the namespace of RestUITest to \Drupal\Tests\restui\FunctionalJavascript and change directory to tests/src/FunctionalJavascript.