Tests that verify documentation is up-to-date ignore subcommands

[keynmol]

The subcommands docs were out of date because the tests only check the output of the tool as a whole, not individual subcommands: https://github.com/sourcegraph/scip/blob/main/cmd/main_test.go#L26

We should ideally check all of it.

Possible approaches:

• Extend current one
• Alternatively, generate the CLI.md entirely out of the tests and use git diff to see if it deviates from what is checked in. Like we do with snapshots

[varungandhi-src]

The test does attempt to check the subcommands. https://github.com/sourcegraph/scip/blob/main/cmd/main_test.go#L54-L56

The problem is that for whatever reason (seems like a bug in urfave/cli, I'm pretty sure this was working before... it doesn't work on an earlier commit too 😵‍💫), when running the subcommand with --help, we get an empty string. This means that the substring check passes.