Changes

fix(tidb): add unit test step for enterprise extensions (#3309)

This pull request includes several changes to the unit test pipelines
for different branches of the `pingcap/tidb` repository. The main focus
is on adding a new stage for testing enterprise extensions.

## Key changes include:

### Addition of Enterprise Extension Testing Stage:
* Added a new stage named 'Test Enterprise Extensions' in multiple
pipeline files to ensure that private submodules are tested. This stage
includes steps to run tests and upload coverage data.
[[1]](diffhunk://#diff-b56f73beeb61b056a6dd2b10e709f4e2c3fb1ce386038e2cba0e9fcaf75bf469R104-R131)
[[2]](diffhunk://#diff-09bf784057b556598a5c45e45288e856c35774a5992096173ee90f949cefff4aL55-R122)
[[3]](diffhunk://#diff-d07b38910eb3c884055beb55a4a6bf94a63c9a6906ef2fd7f9fa4a0bff783d55R104-R131)
[[4]](diffhunk://#diff-fbb8d1f563c1585d02c3bc01ada595ccaec77a4632da766aa08d9f34f4c92a44R100-R127)
[[5]](diffhunk://#diff-0489bf3acebce52e52ca06c3943f8cac4791fa35f8e03f225dac6fb0984a6858R100-R127)
[[6]](diffhunk://#diff-74bed0db67c2a71bbeff8c63856c50c83f9728e05b65c621c11a281541fca667R104-R131)
[[7]](diffhunk://#diff-ec7d8e755ee8101075a0e4049ac2e358e62b26ccd3bb6e5f73e0d3fdf4e1245aR104-R131)

### Affects
- Affects pre-submit type jobs for branches: `release-7.1` ...
`release-8.5`.
- Affects post-submit type jobs for branch: `master`.

## Minor changes include:

### Update to Coverage Upload Path:
* Modified the `prow.uploadCoverageToCodecov` function call to remove
the leading './' from the coverage data file path in various pipeline
files.
[[1]](diffhunk://#diff-b56f73beeb61b056a6dd2b10e709f4e2c3fb1ce386038e2cba0e9fcaf75bf469L78-R78)
[[2]](diffhunk://#diff-d07b38910eb3c884055beb55a4a6bf94a63c9a6906ef2fd7f9fa4a0bff783d55L78-R78)
[[3]](diffhunk://#diff-fbb8d1f563c1585d02c3bc01ada595ccaec77a4632da766aa08d9f34f4c92a44L71-R74)
[[4]](diffhunk://#diff-0489bf3acebce52e52ca06c3943f8cac4791fa35f8e03f225dac6fb0984a6858L71-R74)
[[5]](diffhunk://#diff-74bed0db67c2a71bbeff8c63856c50c83f9728e05b65c621c11a281541fca667L75-R78)
[[6]](diffhunk://#diff-ec7d8e755ee8101075a0e4049ac2e358e62b26ccd3bb6e5f73e0d3fdf4e1245aL78-R78)

### Setting Pull Request Descriptions:
* Added a step to set the pull request description using
`prow.setPRDescription` in the `net-tool` container for several pipeline
files.
[[1]](diffhunk://#diff-fbb8d1f563c1585d02c3bc01ada595ccaec77a4632da766aa08d9f34f4c92a44R39-R41)
[[2]](diffhunk://#diff-0489bf3acebce52e52ca06c3943f8cac4791fa35f8e03f225dac6fb0984a6858R39-R41)
[[3]](diffhunk://#diff-74bed0db67c2a71bbeff8c63856c50c83f9728e05b65c621c11a281541fca667R38-R40)
[[4]](diffhunk://#diff-54a40bd562f68c7c2857bbb6e53d6a159afdbf0189ac861a52bb85d5e25d5c29R38-R40)

### Other Pipeline Adjustments:
* Removed unnecessary timeout options and renamed directories to use
`REFS.repo` instead of hardcoded values in the `merged_unit_test.groovy`
file.
* Added steps to parse flaky test case results and send events to the
cloudevents server in the `merged_unit_test.groovy` file.

---------

Signed-off-by: wuhuizuo <wuhuizuo@126.com>
 (commit: d48a13c)