The lazy evaluation in let() was failing to reset MacOS#full_version
to the original_macos_version. This meant that all tests run after
this one automatically had MacOS#version == 12 which caused some
of the tests to fail if you were running a different macOS version.
- The `brew uninstall` command has `--zap`, so let's make `brew
reinstall` have parity here for a better user experience. (Requested
in issue 12983.)
- It feels weird that to get my new reinstall test to pass I had to add
`--zap` to `cask/cmd/install.rb`, not `cask/cmd/reinstall.rb` to get
the tests to pass. But the `brew reinstall --cask caffeine --zap`
command worked fine all the time. The CLI argument parser from the
test run was complaining about not knowing what `zap` was. As a
result, `--zap` now shows up as a switch in `brew install --help`
which I'm not 100% convinced is the desired UX. But I've edited the
description accordingly to specify that it will only work on
`reinstall` operations (and `--zap` on `install` is a no-op).
```
issyl0 at pictor in /opt/homebrew on reinstall-cask-zap
❯ brew reinstall --cask caffeine --zap
==> Downloading https://github.com/IntelliScape/caffeine/releases/download/1.1.3/Caffeine.dmg
Already downloaded: /Users/issyl0/Library/Caches/Homebrew/downloads/3d6ccfdd3b8d0ab37d1c2468d6e69078c2d31d3b12bf51947c4db21e5f376af2--Caffeine.dmg
==> Implied `brew uninstall --cask caffeine`
==> Backing App 'Caffeine.app' up to '/opt/homebrew/Caskroom/caffeine/1.1.3/Caffeine.app'
==> Removing App '/Applications/Caffeine.app'
==> Dispatching zap stanza
==> Trashing files:
~/Library/Application Support/com.intelliscapesolutions.caffeine
~/Library/Preferences/com.intelliscapesolutions.caffeine.plist
~/Library/Caches/com.intelliscapesolutions.caffeine
~/Library/HTTPStoages/com.intelliscapesolutions.caffeine.binarycookies
==> Removing all staged versions of Cask 'caffeine'
==> Installing Cask caffeine
==> Moving App 'Caffeine.app' to '/Applications/Caffeine.app'
🍺 caffeine was successfully installed!
```
Move the resetting of `MacOS.version` to an `ensure` block.
Fix the tests by adding a test OS and new fixture that has a
different version for that test OS.
