test: add unit tests for data simulation utility functions

[stanley1208]

What does this PR do?

Address the TODO at tests/collections/asr/utils/test_data_simul_utils_asr.py:296 requesting tests for all utility functions by adding 7 new unit tests for 4 previously untested functions in data_simulation_utils.py:

• get_speaker_ids: Test correct speaker ID selection and session index wrap-around via modulo
• build_speaker_samples_map: Test building speaker-to-samples mapping from manifest, including empty manifest edge case
• get_speaker_samples: Test subsetting speaker samples by given speaker IDs
• per_speaker_normalize: Test that output audio RMS matches the target volume level (parametrized over 3 volume values)

All 7 new tests pass locally (32 total in the test class, 0 failures).

Collection: ASR

Changelog

• Add 7 unit tests covering get_speaker_ids, build_speaker_samples_map, get_speaker_samples, and per_speaker_normalize
• Add corresponding imports for the newly tested functions

Usage

python -m pytest tests/collections/asr/utils/test_data_simul_utils_asr.py::TestDataSimulatorUtils -v

[stanley1208]

@nithinraok @pzelasko Ready for review — adds 7 unit tests for 4 untested data simulation utility functions, addressing the TODO at line 296. All tests pass locally. Thanks!

[pzelasko]

CC @tango4j