Extracting testsuite_run from testsuite_file function

In order to run tests from multiple files in the future
iterations, testsuite_run was extracted from testsuite_file.

Now testsuite_list includes the relative path to the provided file
and testsuite_run works by iterating on the testsuite_list output.
Improved test runner to allow targeting directories

Now testsuite can target directories and run multiple test files
at once.

Test files are now required to have the '.test.sh' extension, so
the runner won't pick up non-test files.

testsuite_file was refactored into testsuite_run/testsuite_process
and its the same function for running single files or full dirs.

README file and help text on testsuite_help were modified to reflect
changes.

Test files on the test/ folder are still illustrative. Another suite
was added with basic shell tests to experiment with multiple files.