Fix TestSiteApplicationTest::testUserLogin to properly test user login

Problem/Motivation

There are two todos in testUserLogin that need to be resolved. There isn't an existing issue, so I am making one.

    // @todo This fails. The user-login command uses the one-time login, but
    //   sends us to the install page, which does not set the user cookie.
    $this->assertArrayHasKey('Set-Cookie', $session->getResponseHeaders());

    // Test the user login command with invalid uid.
    // @todo Since we can't successfully test logging in, we also can't test
    //   logging in with an invalid uid. So we assume this is a
    //   false-positive.

Discovered while working on #2962157: TestSiteApplicationTest requires a database despite being a unit test

Steps to reproduce

Proposed resolution

TBD

Remaining tasks

patch
review
commit

User interface changes

API changes

Data model changes

Release notes snippet