Increase the slow test threshold for tests that perform external HTTP requests

[johnbillion]

Tests that perform external HTTP requests will always trigger a slow warning from SpeedTrapListener as core's slow test threshold is set to 150ms.

Let's add a @slowThreshold header to each relevant test with a high value such as 5000ms.

Docs: ​https://github.com/johnkary/phpunit-speedtrap

See #30017.

[johnbillion]

In 38757:

HTTP API: Convert the POST redirect test to use a dataProvider in order for its speed to be more accurately measured.

See #38237

[johnbillion]

After some discussion with Tonya, Juliette, Sergey, and myself, we've decided to remove SpeedTrapListener completely for a few reasons:

• Maintaining compatibility with future versions of PHPUnit becomes more complicated as the listener API changes, particularly with the upcoming support for PHPUnit 8
• The listener has seen little use by anybody - the external HTTP request tests have been running tests longer than the allowed threshold since the day it was introduced
• SpeedTrapListener can still be added as a dependency locally and used when performance investigation is required, it doesn't need to be in core

The removal will be handled in #46149