Add unit test for bpf helper function

These unit tests verify the behavior of NetworkStats helper function. It
uses fake maps to simulate the result of network trafffic stats and
verify the result from helper function is consistent.

Test: compile and run libbpf_test.
Bug: 30950746
Change-Id: I0fcd43e2aedcf7337c6fb78048d9725d18423127
2 files changed