selftests/bpf: Test_progs option for getting number of tests

It can be practial to get the number of tests that test_progs contain.
This could for example be used to create a shell for-loop construct that
runs the individual tests.

Like:
 for N in $(seq 1 $(./test_progs -c)); do
   ./test_progs -n $N 2>&1 > result_test_${N}.log &
 done ; wait

V2: Add the ability to return the count for the selected tests. This is
useful for getting a count e.g. after excluding some tests with option -b.
The current beakers test script like to report the max test count upfront.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/159363985244.930467.12617117873058936829.stgit@firesoul