Add two simple test framework self-tests

Summary:
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.

This diff has purposefully been separated from the stderr checking feature
patch to reduce overall review efforts and keep it focused.

Test Plan:
export SRCDIR=</path/to/your/toplevel/folder>
export PYTHONPATH=$SRCDIR/qa/rpc-tests
sh qa/rpc-tests/test_framework/tests/run_self_tests.sh

You should see output of the self-tests executing successfully.

Reviewers: #bitcoin_abc, deadalnix, sickpig, kyuupichan, awemany

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D237