This commit adds support for test tags. This enables us to specify
an relatively exact subset of tests we want to run based on various
tags such as slow, mining, etc.

Depends on D2158

Run in default mode, only fast tests should run:

./test/functional/test_runner.py

Run with only the slow tag specified. Only slow tests should run:

./test/functional/test_runner.py --tags slow

Run with the legacy "extended" flag. All tests should run.

./test/functional/test_runner.py --extended