test.py: refactor, modularize, cleanup code, make fully OO

- test groups are now separate classes in separate modules
- test data and code is loaded on an as-needed basis
- new TestSuiteRunner and CmdGroupMgr classes
- simplified invocation: if arguments are omitted, all default tests relevant
  for given network and option are run.  The following set of invocations
  provides nearly complete coverage of MMGen's core functionality:

    test/test.py
    test/test.py --segwit-random
    test/test.py --bech32

    test/test.py --coin=ltc
    test/test.py --coin=ltc --segwit-random
    test/test.py --coin=ltc --bech32

    test/test.py --coin=bch
    test/test.py --coin=eth
    test/test.py --coin=etc

MANIFEST.in

@@ -1,6 +1,7 @@
 include README.md SIGNING_KEYS.pub LICENSE INSTALL
 include doc/wiki/using-mmgen/*
 include test/*.py
+include test/test_py_d/*.py
 include test/ref/*
 include test/ref/litecoin/*
 include test/ref/ethereum/*