Enable _testclinic, _testclinic_limited and xxlimited_3_13

The three recent extension modules, `_testclinic`, `_testclinic_limited` and `xxlimited_3_13` are not built on Unix platforms. On Windows the first two are explicitly disabled. On Unix they are disabled implicitly as entires for them do not exist in `cpython-unix/extension-modules.yml` until #1180.

For testing the standard library all three of these are needed for complete coverage. They are not needed for normal Python execution. These would be called if `--pgo-extended` as used for profiling.

These modules should only be included in a subset of the supported Python versions
* `_testclinic` : 3.12+
* `_testclinic_limited`: 3.13+
* `xxlimited_3_13` : 3.15+

Start with cpython-unix/extension-modules.yml and compare the existing extension-module entries with the version requirements in the issue. Add or adjust the entries so _testclinic is enabled for Python 3.12+, _testclinic_limited for 3.13+, and xxlimited_3_13 for 3.15+, while preserving the stated platform restrictions; verify the generated builds include them only in those supported versions.

Written by the indexing model from the issue text.