net: filter: doc: add section for BPF test suite

Mention the recently added test suite in the documentation file.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/filter.txt

@@ -833,6 +833,20 @@ loops and other CFG validation; second step starts from the first insn and
 descends all possible paths. It simulates execution of every insn and observes
 the state change of registers and stack.
 
+Testing
+-------
+
+Next to the BPF toolchain, the kernel also ships a test module that contains
+various test cases for classic and internal BPF that can be executed against
+the BPF interpreter and JIT compiler. It can be found in lib/test_bpf.c and
+enabled via Kconfig:
+
+  CONFIG_TEST_BPF=m
+
+After the module has been built and installed, the test suite can be executed
+via insmod or modprobe against 'test_bpf' module. Results of the test cases
+including timings in nsec can be found in the kernel log (dmesg).
+
 Misc
 ----