unftp-sbe-fs: refactor the integration tests

[asomers]

• Automatically avoid port number collisions
• Factor out duplicate setup code
• Use rstest to separate a parameterized test into multiple independently executable tests. This also eliminates the need to explicitly shut down the server.
• Run every test case in a separate temporary directory.
• Eliminate a sleep in the rename test. I don't think it was ever required.
• Replace excessively long sleeps with polling loops.

Also, Add a test for listing a subdirectory. It's important to make sure that we don't switch from returning relative paths to absolute ones or vice versa.

[robklg]

Thanks!