Move --assert option to subcommand

[Chemaclass]

Context

Right now, -a|--assert is an option from test command, but this does not make sense as this is a command by its own.

Expected

1. Keep -a|--assert option for BC but mark it as deprecated (in the docs?)
2. Create a subcommand bashunit assert that will do the exact same as the current --assert option
3. Update the documentation
4. Add tests for this new command

[cnaples79]

I've opened a PR for this.