Improve getTestServerIfRunning

[tanin47]

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• [x] Have you added tests for any changed functionality?

Helpful things

Fixes

Fixes https://github.com/playframework/playframework/issues/12331

Purpose

Improve the interface of TestServer. Currently, if a TestServer doesn't exist, a few functions raise an exception.

Those functions should have return None (e.g. def runningHttpPort: Option[Int]) or false (e.g. def isRunning: Boolean) instead of raising an exception.