[v6.0.x] AGENTS: add a AGENTS file in ompi/tests

[ompi-backport-bot]

Backport of #14359 to v6.0.x.

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I've been noticing that my agents end up spending a good deal of tokens and money (which I just ran out of) trying to re-figure out how the various parts and pieces work in ompi/tests.

Here's what my agent said about this file it wrote:

Yes, I think a scoped AGENTS.md is a good idea here, and I've created one, following the same pattern as the existing ompi/mpiext/AGENTS.md.

It covers exactly the friction points from the earlier task:

1. A table mapping every ompi/test/ subdirectory to its scope and — critically — how it's actually run (make check vs. make check-abi vs. make check-mpirun vs. "built but not run"), so an agent doesn't have to read a dozen Makefile.ams to figure out spc/monitoring are noinst_PROGRAMS, or that mpirun/ needs an install first.
2. Cross-cutting conventions every simple directory follows (the Makefile.mca-dso-check include, the libmpi+libopen_mpi+libopen-pal LDADD pattern, the "editing Makefile.am only needs make" rule, and the single-process-by-design scope of most directories).
3. A dedicated mpi-abi/ deep-dive — this was the single biggest time sink last time: it explains the case-table → .cbody.in/.prologue.in → generated-C pipeline, the required EXTRA_DIST wiring (the mistake I actually made and had to fix), the optional table keys (requires_feature, skip_exit_codes, etc.), and gives copy-pasteable commands (verified to work, just re-ran them) for iterating on a probe before touching the Makefile, plus the full install-then-check-abi sequence.
4. A decision guide at the end so an agent can quickly pick the right home for a new test instead of defaulting to the wrong directory.

I verified every factual claim in the document against the actual Makefile.ams and against live runs (test_abi_units.py, the probe-rendering snippet, and cross-checked the spc/monitoring "not run by make check" claim against their noinst_PROGRAMS declarations) rather than guessing.:q