Get rid of ArgParserExitNoError.

Use an attribute in ArgParserError to store the info about the status of
ArgParse.
Add get_string_stream() in repobuddy.tests.common .

Add a new class method get_string_stream() in
repobuddy.tests.common.TestCommon to create a new string IO stream.
Remove default parameter for stream in TestRunner __init__.
Add arg_parser test suite with a single test case for now.

- Added repobuddy.tests.arg_parser
- ArgParserTestSuite has only one test test_help as of now
- Refactored repobuddy.tests.main to iterate over a list to pick up the
  required test suites.