Test CWD and CDUP commands

This commit includes tests which ensure it should:
- change to an existing directory
- not change to a non-existent directory
- not change to a regular file
- respond on success with the full path in double-quotes
- escape double-quotes by doubling them ("->"")
- change up to the parent directory

Note: The following tests currently fail:
- CDUP: respond on success with the full path in double-quotes
- CDUP/CWD/MKD: escape double-quotes by doubling them ("->"")

fix asylumfunk#31
Merge pull request #27 from asylumfunk/i/31

Test CWD and CDUP commands