filter pinned test dependencies

[muneebmahmed]

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Addresses #18554. When upgrading formulae, check_install_sanity is called, which seems to walk the dependency tree for a formula to create a full list of dependencies and then - when determining the unsatisfied pinned dependencies - filters out the dependencies tagged as build. A problem with this is that build dependencies might have other non-build dependencies themselves, which won't be tagged as build but will still be present in the dependencies list, despite not being required for the formula being poured from bottle.

In the example from #18554, llvm had been pinned, and was preventing the upgrade of libomp, even though it was not required for the bottle. This was because libomp had a :build dependency on lit, which in turn had a :test dependency on llvm.

This PR just adds an additional check for test before adding to the pinned_unsatisfied_deps list, and in the related issue I suggested it might be better to prune the expansion of build/test dependencies somewhere here.
However, that might more trouble than it's worth for an issue that is not likely to affect many users, especially since there's a simple solution of just unpinning the formulae in question.

[carlocab]

Seems reasonable; thanks!

[MikeMcQuaid]

Makes sense, thanks!