monopam: lint Missing_test is subtree-wide, not per-package
A multi-package subtree like ocaml-cfdp ([cfdp] + [cfdp-eio]) or
ocaml-atp (a dozen lexicon and CLI packages) has shared [test/]
trees whose stanzas don't carry a [(package ...)] annotation, so
there's no way to attribute a given [(test (libraries ...))] to a
specific opam package. Reporting the same missing-test against
every package in the subtree was noisy false-positive city -- e.g.
[cfdp-eio] got blamed for not declaring [alcotest] when [cfdp]'s
test/dune was the actual user.
Move the [Missing_test] check out of [check_package] (which is
called per-opam-package) into the per-subtree loop in [run]. The
new check accepts any package that appears in the union of all
opam packages' declared deps within the subtree, treating the
subtree as a whole.
[Unused_test] stays per-package -- a [:with-test] dep that no
stanza references really is dead weight in that specific opam
file.
Cleared ~60 false-positive missing-test reports; the remaining
~30 are real misses to fix package by package.