perf2cfg_test: run with prebuilt python interpreter

It avoids using python interpreters in local environment, which might
be too old (< 3.8).
After this, searching tests directory no longer works. So import test
modules explicitly in perf2cfg_test.py.

Bug: 326815331
Test: atest system/extras/perf2cfg
Change-Id: I7bec747ec9634c97658585eae3ba81c5c587d1ac
2 files changed