perf test parse-events: Remove cpu PMU requirement

perf test parse-events: Remove cpu PMU requirement

In the event parse string, switch "cpu" to "default_core" and then
rewrite this to the first core PMU name prior to parsing. This enables
testing with a PMU on hybrid x86 and other systems that don't use
"cpu" for the core PMU name. The name "default_core" is already used
by jevents. Update test expectations to match.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>