Resolve scenario files against the scenario directory
Follow-up to #1, which had a bug I missed.
sydtest's typed `scenarioDir` hands the callback a `Path Rel File` **relative
to the scenario directory**. The old `FilePath` version handed one relative to
the working directory, and #1 kept using it that way, so the suite looked for
`test_resources/foo.html` at `foo.html`. All 19 goldens then went missing and
the suite failed with `Golden output not found` rather than with a diff, which
is why this got past a compile.
sydtest's own haddock for `scenarioDirRecur` shows the intended shape:
`readFile (fromRelFile (dir </> rf))`. This does that, binding the directory
so both the read and the golden path go through it.
Joining the directory back on also restores the test descriptions to the paths
they named before this pair of changes, since the description is where the
missing prefix was visible.
### On the red checks
This repo takes sydtest from nixpkgs, which predates typed `scenarioDir`, so
its own CI cannot build either #1 or this until nixpkgs catches up. The failure
in those logs is the `Couldn't match type ... Path Rel Dir / Expected:
FilePath` error against the old signature, not anything about this diff.