Fix extra slash in URI

[MGatner]

Description
Using FeatureTestCase::get('/') currently results in a URI of // due to an extra slash in the URI. This resolves fine during auto-routing but circumvents explicit route definitions such as $routes->get('/', 'Foo::index');.

This PR ensures the slashes are correct going into the URI regardless of baseUrl and $path.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• n/a User guide updated
• Conforms to style guide