le_audio/client: Reset mock pointers in tests

In some cases when AcquireUnicast was not called the test
was still the owner of these pointers so we need to delete them in TearDown.

Bug: 266872728
Test: atest bluetooth_le_audio_client_test
Tag: #feature

Change-Id: I4998c081ff1869925c9adc8077f90e043142fdbf