Improve subtest selection logic when using -t/-a/-d parameters.
In particular, more than one subtest can be specified or a
combination of tests / subtests.

-a send_signal -d send_signal/send_signal_nmi* - runs send_signal
test without nmi tests

-a send_signal/send_signal_nmi*,find_vma - runs two send_signal
subtests and find_vma test

-a 'send_signal*' -a find_vma -d send_signal/send_signal_nmi* -
runs 2 send_signal test and find_vma test. Disables two send_signal
nmi subtests

-t send_signal -t find_vma - runs two *send_signal* tests and one
*find_vma* test

This will allow us to have granular control over which subtests
to disable in the CI system instead of disabling whole tests.

Also, add new selftest to avoid possible regression when
changing prog_test test name selection logic.
Signed-off-by: Mykola Lysenko <mykolal@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220409001750.529930-1-mykolal@fb.com