[CI] Review usage of check_BE function in split rendering pytest

Discovered while debugging !2173 (merged): the split rendering pytest invokes the old check_BE() python function if get_mld is not specified.

This can be slightly misleading because check_BE() attempts to delay align the inputs before the check, which means that a pure delay change will not be detected as nonBE. This function is still used for the HRTF binary loading testsuite and compare_renderer_args() (used for the refvec vs OTR tests).

This should be phased out for split rendering and replaced with cmp_pcm().