non-blocking check_axi_stream does not respect stall_config

[eschmidscs]

Symptom
Using check_axi_stream with blocking=>false and a non-zero stall_config keeps TREADY high for all time.

Cause
The stall_config works fine for all pop*-functions.
However, the check_axi_stream with blocking => false is not based on pop functions, but issues a specific message to the slave. The implementation of that part does not respect the stall_config. It is simply not implemented.

[eschmidscs]

I'm working on a test and fix now.

[eschmidscs]

Fixed with #720