Test the strictness analyzer using annotations

This adds a new directory, tests/stranal/sigs. Tests therein are
expected to use the StrAnalAnnotation GHC plugin (also therein) to
annotate (some of) their top level functions like this:

    foo x = x
    {-# ANN foo (StrAnal "<S,1*U>") #-}

Then the test will fail if the strictness analyzer finds a different
strictness signature.
Add test case for #8569