Update make tests to run tests per the README, remove unused test runners

[jbirddog]

Currently on main make tests fails with:

$ make tests
cd tests/SpiffWorkflow; \
	[ -e run_suite.* ] && ./run_suite.* || [ ! -e run_suite.* ]
/bin/sh: 2: ./run_suite.py: Permission denied
make: *** [Makefile:39: tests] Error 1

This change updates the Makefile so that make tests runs the unit tests in the same way that is described in the README:

$ make tests
cd tests/SpiffWorkflow
coverage run --source=SpiffWorkflow -m unittest discover -v . "*Test.py"
...

----------------------------------------------------------------------
Ran 528 tests in 39.810s

OK (skipped=2)

Also removes run_suite.sh and run_suite.py files that appear to be legacy.