selftests/bpf: Fix use of uninitialized op_name in log tests

One of the test assertions uses an uninitialized op_name, which leads
to some headscratching if it fails. Use a string constant instead.

Fixes: b1a7a480 ("selftests/bpf: Add fixed vs rotating verifier log tests")
Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230413094740.18041-1-lmb@isovalent.com