Reorganize tests into named testsets and cover UDF value marshalling (#270) (#351)

* test: group runtests.jl into named testsets (#270)

Pure reorg: wrap the flat test file in named @testset blocks with no
assertion changes except aligning the strict Symbol-in-TEXT expectation
with upstream #349 (multi-row type scan).

* test: cover UDF value marshalling round-trips (#270)

Add a dedicated testset for scalar UDF bind/round-trip coverage including
Int64 boundaries, Float64, String, blob, and NULL.

Author: JohnCobbler