KTOR-7682 Fix TestApplication.stop() doesn't stop the application anymore

[osipxd]

Subsystem
Test Infrastructure

Motivation
KTOR-7682 TestApplication.stop() doesn't stop the application anymore

Solution
The problem occurs because TestApplication.client uses an instance of an application different from the returned by TestApplication constructor-like function and calls start() on it. TestApplication.start() calls TestApplicationEngine.start() under the hood which rewrites cancellationJob, so coroutine waiting for cancellation never ends.

To fix this problem:

• make TestApplication return application from builder – the same as used in a client
• remove builder from TestApplication constructor to make builder the only way to get TestApplication and break circular dependency: TestApplication -> ApplicationTestBuilder -> TestApplication

It is better to review commit-by-commit

[bjhham]

Nice!