selftests/bpf: shorten subtest names for struct_ops_module test

Drive-by clean up, we shouldn't use meaningless "test_" prefix for
subtest names.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20240507001335.1445325-8-andrii@kernel.org


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>