testsuite: rewrite the shell testsuite in Python

testsuite: rewrite the shell testsuite in Python

Replace the entire shell-based testsuite with Python. runtests.py
already drove the suite (it had replaced runtests.sh earlier); this
converts all 60 test scripts from *.test shell to *_test.py and adds
testsuite/rsyncfns.py as the shared helper module -- the Python
counterpart of the now-removed rsync.fns.

runtests.py:
  * Discovers and runs both *.test and *_test.py; dispatches the
    Python tests via the same python3 that runs the harness.
  * Extends PYTHONPATH so tests can `import rsyncfns`.

testsuite/rsyncfns.py provides everything the ports need:
  * environment wiring (scratchdir / srcdir / TOOLDIR / RSYNC /
    TLS_ARGS, and HOME pointed at the per-test scratch dir);
  * result reporting -- test_fail / test_skipped / test_xfail mapping
    to the 0 / 1 / 77 / 78 exit-code convention;
  * the transfer-and-verify helpers checkit, checkdiff, verify_dirs,
    rsync_ls_lR, check_perms and the v_filt output filter;
  * fixture builders hands_setup, build_symlinks, build_rsyncd_conf,
    make_data_file, cp_p / cp_touch, makepath / rmtree.

All 60 tests are converted, including the four split-variant tests
that share one source via a Makefile-built symlink (chown/chown-fake,
devices/devices-fake, xattrs/xattrs-hlink, exclude/exclude-lsh);
Makefile.in's CHECK_SYMLINKS now points at the *_test.py names.

The dead rsync.fns shell library is removed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>