also fix typo in `--root-url` option

These were all made `--formulae` and `--casks` for consistency but that
doesn't behave as expected because they have special behaviours in
`args`. Instead, use the singular form everywhere.

This allows auditing a single tap (so is useful for `test-bot`).

- Ensure the usage documentation always mentions casks when appropriate
- Use --formulae or --casks consistently
- Provide --formulae and --casks switches to upgrade
- Mark more switches as conflicting

This should allow better handling flaky tests.

This should help with cases like
https://github.com/Homebrew/homebrew-bundle/issues/751
and is good practise in general.

Also, document the `brew update --preinstall` flag that is being used
here so others can run it manually e.g. as part of CI if needed.

- make `brew readall` only check Homebrew/brew syntax with no tap
  passed.
- pass specified tap to `brew readall` test for speed
- only shell out once in `FormulaInfo` test for speed

Co-authored-by: Eric Knibbe <enk3@outlook.com>