perf test: Rename functions and variables for better clarity

The relationship between subtests and test cases is somewhat
confusing, so let's do away with the notion of sub-tests and switch to
just working with some number of test cases. Add a
test_suite__for_each_test_case as in many cases, except the special
one test case situation, the iteration can just be on all test
cases. Switch variable names to be more intention revealing of what
their value is.

This work was motivated by discussion with Kan where it was noted the
code is becoming overly indented:
https://lore.kernel.org/lkml/20241109160219.49976-1-irogers@google.com/
Unifying more of the sub-test/no-sub-tests avoids one level of
indentation in a number of places.

Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Link: https://lore.kernel.org/r/20250110045736.598281-2-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: captain5050