selftests/bpf: allow requesting log level 2 in test_verifier

selftests/bpf: allow requesting log level 2 in test_verifier

Log level 1 on successfully verified programs are basically equivalent
to log level 4 (stats-only), so it's useful to be able to request more
verbose logs at log level 2. Teach test_verifier to recognize -vv as
"very verbose" mode switch and use log level 2 in such mode.

Also force verifier stats regradless of -v or -vv, they are very minimal
and useful to be always emitted in verbose mode.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221005161450.1064469-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>