selftests/bpf: remove logic duplication in test_verifier

test_verifier tests can specify single- and multi-runs tests. Internally
logic of handling them is duplicated. Get rid of it by making single run
retval/data specification to be a first run spec.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Cc: Krzesimir Nowak <krzesimir@kinvolk.io>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>