perf test: Avoid use perf_env

perf test: Avoid use perf_env

The perf_env global variable holds the host perf_env data but its use
is hit and miss. Switch to using local perf_env variables and ensure
scoped perf_env__init and perf_env__exit. This loses command line
setting of the perf_env, but this doesn't matter for tests. So the
perf_env is fully initialized, clear it with memset in perf_env__init.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250724163302.596743-13-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>