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).
Add a Function::display() method which can include ISA-specific
information when printing the function.
If a test file has a unique ISA, use that in the `test cat`
implementation.
