Resolve relative test-selection args before matching rsync roots

[agu2347]

Fixes #971.

make_reltoroot() (used to translate a test-selection arg like tests/test_sample.py into a path relative to an rsync root before sending it to a remote worker) constructed fspath = Path(parts[0]) directly from the given arg. py.path.local, used here prior to the project's migration to pathlib, transparently resolved a relative path against the current working directory as part of its own construction; plain pathlib.Path does not do this.

As a result, a relative path given on the command line (e.g. running pytest -d --tx socket=... tests/test_sample.py from the project root) never compared as a match or subpath of any of the (absolute) rsync roots via relative_to(), even when it does in fact point inside one of them once resolved against the cwd -- raising "arg ... not relative to an rsync root" and breaking test selection entirely for any relative path, which worked correctly before the pathlib migration -- exactly as diagnosed in the issue, including the exact commit that introduced the regression.

Fix: resolve fspath explicitly (Path.resolve()) before the existence check and root-matching loop, restoring the old py.path.local behavior.

Testing: verified directly against the exact reported scenario: calling make_reltoroot() with a relative path arg and an absolute root previously raised the exact "not relative to an rsync root" error; with the fix, it resolves correctly. Also verified: an absolute path arg (the case that already worked) is unaffected; an arg with a "::test_id" suffix is preserved correctly; and a non-existent relative path still passes through unchanged rather than raising.

Added regression tests covering all of the above using pytester's tmp_path/monkeypatch.chdir, matching this test file's existing style. I confirmed the relative-path tests fail with the original code (reproducing the exact reported ValueError) and pass with the fix. Ran the full existing test_workermanage.py suite (23 passed: 19 baseline + 4 new; 4 pre-existing, unrelated warning-serialization failures confirmed identical with and without this change via a fixed random seed on a clean checkout).