Add coverage test

[afunTW]

Summary

Under the pytest framework with pytest-cov module, the output will be like the following message

---------- coverage: platform linux, python 3.8.10-final-0 -----------
Name                         Stmts   Miss  Cover
------------------------------------------------
proxybroker/__init__.py         22      0   100%
proxybroker/api.py             229    196    14%
proxybroker/checker.py         209    185    11%
proxybroker/errors.py           24      0   100%
proxybroker/judge.py            78     60    23%
proxybroker/negotiators.py      96      4    96%
proxybroker/providers.py       358    215    40%
proxybroker/proxy.py           217     58    73%
proxybroker/resolver.py         93     12    87%
proxybroker/server.py          245    218    11%
proxybroker/utils.py            83     22    73%
tests/__init__.py                0      0   100%
tests/conftest.py               49      6    88%
tests/test_negotiators.py       58      0   100%
tests/test_proxy.py            173      0   100%
tests/test_resolver.py          65      0   100%
tests/test_utils.py             24      0   100%
tests/utils.py                   8      0   100%
------------------------------------------------
TOTAL                         2031    976    52%

[afunTW]

I figure out that the CI flow all depends on poetry.lock, which should be define in CONTRIBUTE.md. The Ci failed because I updated the requirements/ only

[bluet]

oops tests are failing https://github.com/bluet/proxybroker2/runs/7989566629?check_suite_focus=true#step:9:14

pytest: error: unrecognized arguments: --cov

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[bluet]

LGTM

[hms5232]

I figure out that the CI flow all depends on poetry.lock, which should be define in CONTRIBUTE.md. The Ci failed because I updated the requirements/ only

related #93 (comment)