perf: clean up tests

The regular and overwritable perf buffer tests currently don't share
much test scaffolding because we want to know which sample we've read
for the overwritable case.

Reduce code duplication by adopting the regular and overwritable tests
to use a common sample format which includes both size and sample ID.
Also adopt some of the helpers to fit their most common use case.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>