Update the UT for SignalStrengthUpdateRequest

Make sure test cases cover following scenarios:
- Request allows same SignalMeasurementType on different RAN
- Request allows different SignalMeasurementTypes on same RAN
- IAE should thrown when SignalMeasurementType on same RAN
  is not unique

Bug: 198318496
Test: atest SignalStrengthUpdateRequestTest
Change-Id: Ic1941730f9833219f0661d32722aabb1882277ac