Improve detecting available test modules in test package

By using glob and path.splitext instead of manually iterating over files
in the directory and filtering out files based on their names.
Removed running tests from setup.py

tests package has a better of finding test modules. Test modules use
relative imports, hence require to be run by another runner script
instead of directly being executed.
Adds run-tests script to run unittests

This runner script would import target tests and runs them.