Implement fully automated end-to-end CI testing workflow

[weiiwang01]

Updated the github-runner charm to include a new 'test mode'. This functionality facilitates the charm's deployment on an LXD container cloud, using LXD nesting. Building upon this 'test mode', we have also developed a new e2e test that is fully automated, and operable on GitHub hosted runners. This also resolves the bootstrap problem that we had encountered with our previous e2e test configuration, which requires a self-hosted runner to test self-hosted runner.

[gregory-schiano]

I'm not a big big fan of the "test-mode" config parameter just for the E2E tests, but to be honest couldn't think about an good alternative for now

[github-actions]

Test coverage for ccd3aff

Name                                    Stmts   Miss Branch BrPart  Cover   Missing
-----------------------------------------------------------------------------------
src/charm.py                              314     58     68     14    80%   127-129, 260-270, 286-288, 297-298, 305, 316, 327->344, 332-337, 354-355, 359-360, 365-368, 384-385, 419-427, 457-459, 462->exit, 466-468, 491-496, 506-507, 509-510, 512-513, 590-591, 601
src/errors.py                              18      0      0      0   100%
src/event_timer.py                         42      8      4      0    74%   96-99, 119-122
src/github_type.py                         36      0      0      0   100%
src/lxd_type.py                            34      0      2      0   100%
src/repo_policy_compliance_service.py       5      5      0      0     0%   4-14
src/runner.py                             215     37     58     19    79%   39->43, 143-151, 162-163, 170-177, 183-189, 244-249, 254, 291-293, 300, 314, 324, 328, 330, 345, 379-384, 394, 478, 511, 537, 542-554, 568, 586
src/runner_manager.py                     196     20     80      8    88%   202-203, 217-223, 227-229, 239-240, 293, 317-321, 340, 358, 472, 492
src/runner_type.py                         46      0     10      0   100%
src/utilities.py                           55      2     12      4    91%   74->76, 78->84, 89-91
-----------------------------------------------------------------------------------
TOTAL                                     961    130    234     45    84%

Static code analysis report

Run started:2023-06-21 05:48:52.061899

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2082
  Total lines skipped (#nosec): 0
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 9

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):