test_flowgraph: add tests for SinkI32.cpp

Add inline tests that are standalone and do not rely
on the flowgraph code.
Cleanup 16-bit conversion tests and shared data with 32-bit tests.

Bug: 232744945
Test: atest test_flowgraph
Change-Id: I670ebaa6b81a4c2fbd83464542b49f8919be561d