IVGCVSW-3904 Add more unit tests for send thread with BufferManager

IVGCVSW-3904 Add more unit tests for send thread with BufferManager

 * Add timeout parameter to wait for readable data
 * Write all readable data to the profiling connection when ready to read
 * Set ready to read when buffer exhaust
 * Ensure that readable data get written to profiling connection
   before the send thread is stopped
 * Add MockWriteProfilingConnection to be able to test WritePacket
 * Refactor BufferManager and the unit tests

Signed-off-by: Narumol Prangnawarat <narumol.prangnawarat@arm.com>
Change-Id: I80ae01bd8d0119a3a3a957069ae8ac521c005a12