use is_ci_test where appropriate

[smoors]

is_ci_test is simpler than having to set both bench_name and bench_name_ci.
it's also easier to understand and to explain :)

bench_name and bench_name_ci are still used is some test, i'm wondering if maybe we should remove it completely? (will be for another PR in any case)

see also the companion docs PR:

• use is_ci_test instead of bench_name and bench_name_ci docs#623