Anyone know the file format?
The NIC-301 or NIC-400 AXI interconnects are just verilog RTL code, so you can simulate them in your system as you would any other piece of verilog IP, and the test file format is then determined by your testbench, not the NIC design.
But of you are asking about the testbenches generated when you render a NIC-301 or NIC-400 design, these testbenches and test files are not meant to be reused or modified in any way, so the test file format is not documented in the NIC deliverables. It is however based on the text file format used in Arm's AXI File Reader product, BP144, but adapted to include extra fields required for more complex AXI transactions.
The NIC supplied tests do simple connectivity checks to demonstrate basic functionality, ensuring that each source can access each connected destination, and where you have a GPV there are tests to verify that each GPV register can be accessed, but the expectation would be that more application specific tests would be performed once you have integrated the NIC logic into your design, and so the test file format then used is that used by your own testers.