testsuite: More robust library way detection

Previously `test.mk` would try to determine whether the dynamic,
profiling, and vanilla library ways are available by searching for
`PrimOpWrappers.{,dyn_,p_}hi` in directory reported by `ghc-pkg field
ghc-prim library-dirs`. However, this is extremely fragile as
there is no guarantee that there is only one library directory. To
handle the case of multiple `library-dirs` correct we would
have to carry out the delicate task of tokenising the directory list (in
shell, no less).

Since this isn't a task that I am eager to solve, I have rather moved
the detection logic into the testsuite driver and instead perform a test
compilation in each of the ways. This should be more robust than the
previous approach.

I stumbled upon this while fixing #20579.