tests: drivers: audio: dmic_api: Enable coverage calculation

When test is run with enabled coverage, additional RAM is required
to store coverage data.
Digital Microphone test is reserving most of the available RAM
to store audio samples. Thus, test fails to build with coverage
enabled due to RAM overflow.

Decrease size of the audio buffer when coverage is enabled.

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>