use testconfig in hack/test script

[crazy-max]

https://github.com/moby/buildkit-bench/blob/5593f8d7f160ae17a5462bee02af395f905e4355/hack/test runs benchmarks with default values that might not be suitable with testconfig: https://github.com/moby/buildkit-bench/blob/master/testconfig.yml

This script should use the testconfig if bench regexp, count and time are not set so it runs like ci.