pyproject: exclude tests from coverage

[jelly]

With these changes:

varlink/tests/test_basic_network.py ....                                                                                                                             [ 21%]
varlink/tests/test_certification.py ..                                                                                                                               [ 31%]
varlink/tests/test_mocks.py .....                                                                                                                                    [ 57%]
varlink/tests/test_orgexamplemore.py .                                                                                                                               [ 63%]
varlink/tests/test_scanner.py .....                                                                                                                                  [ 89%]
varlink/tests/test_server_encoding.py ..                                                                                                                             [100%]

============================================================================== tests coverage ==============================================================================
_____________________________________________________________ coverage: platform linux, python 3.13.7-final-0 ______________________________________________________________

Name                  Stmts   Miss  Cover
-----------------------------------------
varlink/__init__.py      23     14    39%
varlink/client.py       355    140    61%
varlink/error.py         59     16    73%
varlink/mock.py         147     16    89%
varlink/scanner.py      285     34    88%
varlink/server.py       285     75    74%
-----------------------------------------
TOTAL                  1154    295    74%
============================================================================ 19 passed in 7.27s ============================================================================