Created a new test_framework/tests/ subdirectory for test framework
self-tests.
A test runner for these tests is provided in `run_self_tests.sh`.
This is not integrated into routine test suite execution targets,
it must be run manually for now.
It is possible to call these self-tests directly provided your
PYTHONPATH is to the location of your rpc-tests/ folder, and your
SRCDIR is set to the parent of the top level src/ folder, so that
tests can find the bitcoind executable.
Test framework self-tests are just regular Python tests in the same
manner as the Bitcoin regression tests.
The two first tests were used during development of internal framework
changes for ABC.
Note: Depends on D230
This diff has purposefully been separated from D230 to reduce overall
review efforts and keep it focused.