Closes #122 - split up unit and integration test runners and coverage

[ascopes]

• Use surefire for unit tests in main module.
• Use failsafe for integration tests in main module.
• Use separate JaCoCo report for unit tests.
• Use separate JaCoCo report for integration tests.
• Rework mutation test skippage annotation in main module.
• Instruct JaCoCo to run on verify phase as well as test phase.
• Implement codecov feature flags for integration and unit tests individually.
• Skip running test runner at all for Manifold when on <jdk>(11,)</jdk> similar to what Spring does for <jdk>[,15)</jdk>
• Rewrite parts of prepare-test-outputs-for-merge.sh to cope with new report locations.

Closes #122.

[github-actions]

Test Results

  1 302 files   -   13    1 302 suites   - 13   4s ⏱️ ±0s
17 138 tests  -   46  17 062 ✔️  -   46  76 💤 ±0  0 ❌ ±0 
18 796 runs   - 126  18 720 ✔️  - 126  76 💤 ±0  0 ❌ ±0 

Results for commit 4b86d22. ± Comparison against base commit e633882.

[codecov-commenter]

Codecov Report

Merging #217 (1ec5e6b) into main (e633882) will not change coverage.
The diff coverage is n/a.

❗ Current head 1ec5e6b differs from pull request most recent head 05c366b. Consider uploading reports for the commit 05c366b to get more accurate results

@@           Coverage Diff           @@
##             main     #217   +/-   ##
=======================================
  Coverage   67.68%   67.68%           
=======================================
  Files          70       70           
  Lines        2448     2448           
  Branches      226      226           
=======================================
  Hits         1657     1657           
  Misses        723      723           
  Partials       68       68           

[ascopes]

Spotted and reported fabriciorby/maven-surefire-junit5-tree-reporter#29 as part of this PR, but this is not impacting builds currently.