Modify test runners to be a class-based structure

[Russell Keith-Magee]

When test runners were originally introduced, the run_tests() method was fairly simple, so users who needed custom test execution behavior didn't have a major hurdle to overcome.

However, over time, the test runner has become more complex, in order to support more complex features. "Just write a custom test runner" isn't as easy as it once was.

This is especially problematic for someone that wants a relatively simple change - e.g., enabling DEBUG during testing, or using a different test discovery method, or using a different TestRunner class to execute the suite.

The test runner should be a class with overridable functions, rather than a single function.

[Russell Keith-Magee]

First draft at class-based test runners.

[Russell Keith-Magee]

(In [12255]) Fixed #12624 -- Modified test runners to be class based.