Fix cask dependency discovery

[muneebmahmed]

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Fixes Homebrew/homebrew-cask#152726.
This is an issue that affects only casks that have a depends_on formula clause, and only when that formula has uninstalled test dependencies somewhere in its tree. In this situation, brew when installing the cask will also install the test dependencies, even though they are not needed. Not many casks have a depends_on formula clause, so I don't expect the issue to be very common; so far I've seen this issue when installing the casks neovide and goneovim.

I did a search for the function this commit modifies, and it seems to only be used in upgrade.rb and cask/installer.rb, which would explain why the issue was observed only when installing casks and not formulae. I tested brew reinstall --cask neovide with this commit and did not observe the installation of the extraneous dependencies as in Homebrew/homebrew-cask#152726, so this should fix that issue.

Although this did not fail the brew style check, I'm not entirely sure if this fix matches Homebrew's style, so please let me know if an alternate fix is preferred.

[MikeMcQuaid]

Great catch and thanks again for the PR, @muneebmahmed!