Migrate unit tests to use Go workspace

[ivanvc]

Introduce the new functions: run_go_tests and run_go_test_expanding_packages, which implement (without Shellcheck exceptions) the old behavior from go_test, simplifying the arguments received.

Replaced the three possible values mode with:

1. keep_going: By using the KEEP_GOING_TESTS environment variable. This mode expanded the relative modules; it can be run by using run_go_tests_expanding_packages.
2. parallel: It worked without expanding the relative modules with the absolute modules. Therefore, this mode can be run directly using run_go_tests.
3. fail_fast: By adding an argument "-failfast", and not setting the KEEP_GOING_TESTS environment variable.

The argument flags_for_package_func can be replaced by regular arguments passed to the function.

These two changes simplify the complexity of the go test wrapper function and allow further customization in the callers.

The JUnit XML generation works the same way, by reading the JUNIT_REPORT_DIR or ARTIFACTS environment variable. It generates the same reports.

Temporarily introduce the get_junit_filename_prefix function, as a replacement for junitFilenamePrefix. The latter will be removed once all test targets are using the new functions.

Finally, update the unit tests function to make use of the new functions, and allow running the tests by using the workspace (i.e., there's no need to change directories anymore).

Part of #18409.

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.

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[ivanvc]

/test all

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.11%. Comparing base (31fbd99) to head (cef8274).

Additional details and impacted files

see 20 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #20872      +/-   ##
==========================================
- Coverage   69.20%   69.11%   -0.10%     
==========================================
  Files         422      422              
  Lines       34831    34831              
==========================================
- Hits        24104    24072      -32     
- Misses       9328     9354      +26     
- Partials     1399     1405       +6     

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[ivanvc]

/retest