Create integration tests for "rmdir" utility

[jgarzik]

The rmdir utility found in tree/src/rmdir.rs lacks integration tests.

Create a new set of integration tests in tree/tests/rmdir-tests.rs. Test both with and without -p. Ensure negative tests exist, testing failure conditions, as well as positive tests. Test with zero, one and multiple pathnames on the command line. Verify that the utility CLI exit code is POSIX compliant and correct in all cases.