Fix FTP test leaving behind temporary files

[srenfo]

Fixes two bugs in the FTP tests:

1. test_appe was leaving behind temporary files in conpot/tests/data/data_temp_fs/ftp
2. test_help contained an assert that AFAICT was always true (the one marked "test command specific help")

Since I was already looking at the file I also refactored some test initialization code.

Caveat: I saw some occasional failures in the FTP tests which looked like race conditions to me, where one test would sometimes find a file another test had created. I don't think my particular changes are to blame but I can't be sure.

[coveralls]

Pull Request Test Coverage Report for Build 1268

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 25 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.1%) to 73.951%

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Files with Coverage Reduction	New Missed Lines	%
conpot/utils/greenlet.py	1	97.56%
conpot/protocols/http/command_responder.py	2	65.57%
conpot/protocols/ftp/ftp_handler.py	10	81.58%
conpot/protocols/ftp/ftp_base_handler.py	12	83.08%

Totals
Change from base Build 1253:	0.1%
Covered Lines:	5570
Relevant Lines:	7532

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💛 - Coveralls

[glaslos]

LGTM. Nice cleanup and refactor.