r_submix: Add some tests

Add simple tests to verify remote submix behavior regarding
blocking writes. Currently one of the tests fails (does not finish),
this needs to be fixed.

Also fixed some minor issues in remote submix module code.

Bug: 73175392
Test: r_submix_tests
Change-Id: Ic88d0c385c0102e35b4f751f9c5cd8a6488949c8
3 files changed