cask/test: create cask installer stub

[bevanjkay]

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This PR creates a new test method for using an installer stub for cask installs.
There are a number of tests that require an "installation" to exist but they don't actually make use of the installed files. The stub method creates a minimal installation that is detected for use in these instances.

The cask/upgrade test suite time has been reduced by 55% according to my testing.

Note: I use claude-code to help with the methodology for creating this new method. I had tried to optimise this test a few times unsuccessfully in the past, but this approach makes a lot of sense to me.

[MikeMcQuaid]

Good idea, thanks!