TMGimporter: port the libtmg OS-test split (PR #949) to maintenance/gramps61

[eduralph]

Summary

Upstream addons-source PR 949 split TMGimporter/tests/test_libtmg.py into
pure-function tests (test_libtmg.py, every OS) and DB-backed tests
(test_linux_libtmg.py, Linux-only via the test_linux_* filename convention)
and merged to maintenance/gramps60. The maintainer's cherry-pick forward
to maintenance/gramps61 failed, and he asked for a separate gramps61 PR:

Cherry picking this to gramps61 failed. Need a new PR for the changes for gramps61.
— @GaryGriffin

Why the cherry-pick fails

maintenance/gramps61's test_libtmg.py has independently diverged from
the gramps60 version that split was cut against. gramps61 added, in exactly the
regions the split rewrites:

1. an inline gi.require_version("Gtk"/"Gdk", "3.0") + SkipTest block at the top, and
2. a sys.platform == "win32" skip inside the shared _make_db() helper.

The patch's context lines (the old _make_db body, import tempfile) no longer
match, so git cannot replay it.

Target state — converge gramps61 onto gramps60's post-split structure

Make TMGimporter/tests/ on maintenance/gramps61 identical to
maintenance/gramps60 after the split:

• test_libtmg.py → pure-function tests only (gramps60 blob 4851746a).
• test_linux_libtmg.py → DB-backed tests + helpers (gramps60 blob 35bd117).
• tests/__init__.py stays empty (already matches gramps60).

This drops the two gramps61-only stopgaps:

• inline GTK pin — GTK-version pinning is not an individual test file's
  job. It is owned by the central repo-root tests/__init__.py (the central
  repo-root pin, present on both branches), by Gramps' own GUI launcher, and —
  for per-addon invocation — by the planned per-addon tests/__init__.py
  migration (Move inline gi.require_version pins into each addon's tests/__init__.py #38). Under the testbed harness the GI versions are also pinned by
  the gi_bootstrap sitecustomize. gramps60's merged files carry no inline pin.
• win32 skip in _make_db() — superseded by the file split: the DB tests
  now live in test_linux_*, which the Windows lane excludes, so the runtime
  skip is redundant. gramps60's merged test_linux_libtmg.py carries no
  win32 skip and is green on the maintainer's CI.

Out of scope

• Does not close TMGimporter: real-DB import tests skipped on Windows (conda lane can't run matching gramps) #32 — the Windows real-DB coverage gap remains open until
  gramps 6.1 lands on conda-forge; this only brings gramps61's mechanism in
  line with gramps60.
• Does not do Move inline gi.require_version pins into each addon's tests/__init__.py #38 — migrating the GTK pin into TMGimporter/tests/__init__.py
  is Move inline gi.require_version pins into each addon's tests/__init__.py #38's coordinated, repo-wide rollout (depends on the empty __init__.py
  groundwork), not this PR.

Verification

• The gramps60 split is merged and green on the maintainer's CI (he is asking to
  port it, not revert) — proving the test_linux_* Windows-exclusion works and
  that dropping both stopgaps is the production-correct shape.
• Testbed Linux runner (engine/scripts/ubuntu/run-addon-unit.sh) runs
  test_linux_* on Linux (excludes only test_windows_*), so the DB tests run
  against the matching gramps 6.1.

References

• Windows-coverage tracking issue: TMGimporter: real-DB import tests skipped on Windows (conda lane can't run matching gramps) #32
• GTK-pin placement direction: Move inline gi.require_version pins into each addon's tests/__init__.py #38