selftests/bpf: Introduce xsk statistics tests

This commit introduces a range of tests to the xsk testsuite
for validating xsk statistics.

A new test type called 'stats' is added. Within it there are
four sub-tests. Each test configures a scenario which should
trigger the given error statistic. The test passes if the statistic
is successfully incremented.

The four statistics for which tests have been created are:
1. rx dropped
Increase the UMEM frame headroom to a value which results in
insufficient space in the rx buffer for both the packet and the headroom.
2. tx invalid
Set the 'len' field of tx descriptors to an invalid value (umem frame
size + 1).
3. rx ring full
Reduce the size of the RX ring to a fraction of the fill ring size.
4. fill queue empty
Do not populate the fill queue and then try to receive pkts.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://lore.kernel.org/bpf/20210223162304.7450-5-ciara.loftus@intel.com