tests: Bluetooth: Audio: Remove uses of K_FOREVER in syswg for TX

Several tests were using K_FOREVER when allocating the
buffer for TX in the system workqueue, which is illegal behavior.

The solution chosen was to create a TX thread to handle TX,
similar to the solution used in the audio shell and some
sample applications.

This way we can continue to use K_FOREVER when allocting buffers
and it will always be done in a round-robin fashion while
TXing as much as possible, by always enqueuing all the buffers
with mock data.

Since this works for all streams (both broadcast and unicast),
it was obvious to use the same implementation for all tests,
and thus cleaning up the tests a bit and more them more similar.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>