fix(ci_visibility): properly resolve decorated functions for test source file information [backport 2.9]

Backport 203e89489f1e698a127317dc57b01e9d6467e859 from #9586 to 2.9.

Fixes an issue where decorated test functions could resolve to the wrong location when certain decorators (eg: @mock.patch) were used.

This also fixes the fact that source file info paths might not always be relative to the current repo root.

The test added in this PR verifies the above by using a variety of decorators as well as executing from the nested_dir directory instead of the git repo root, with the source file info properly showing the path as nested_dir/test_mydecorators.py.

Checklist

[x] Change(s) are motivated and described in the PR description
[x] Testing strategy is described if automated tests are not included in the PR
[x] Risks are described (performance impact, potential for breakage, maintainability)
[x] Change is maintainable (easy to change, telemetry, documentation)
[x] Library release note guidelines are followed or label changelog/no-changelog is set
[x] Documentation is included (in-code, generated user docs, public corp docs)
[x] Backport labels are set (if applicable)
[x] If this PR changes the public interface, I've notified @DataDog/apm-tees.

Reviewer Checklist

[x] Title is accurate
[x] All changes are related to the pull request's stated goal
[x] Description motivates each change
[x] Avoids breaking API changes
[x] Testing strategy adequately addresses listed risks
[x] Change is maintainable (easy to change, telemetry, documentation)
[x] Release note makes sense to a user of the library
[x] Author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
[x] Backport labels are set in a manner that is consistent with the release branch maintenance policy

DataDog / dd-trace-py

fix(ci_visibility): properly resolve decorated functions for test source file information [backport 2.9] #9644

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