Refactor the filetests harness so that it can be run as part of
`cargo test`. And begin reorganizing the test harness code in preparation
for moving it out of the src directory.
- Test subcommand files are now named `test_*.rs`.
- cton-util subcommand files now just export their `run` and nothing else.
- src/filetest/mod.rs now also just exports `run` and nothing else.
- Tests are now run in release mode (with debug assertions enabled).
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