run_tests: make the script autodetect and run all tests, allow overrides
run_tests: make the script autodetect and run all tests, allow overrides

BUG=none
TEST=run all tests

Change-Id: Id341e00e79e2bd734757519ef93a47e5907ce505
Reviewed-on: https://gerrit.chromium.org/gerrit/32033
Commit-Ready: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Reviewed-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>
Tested-by: Zdenek Behan <483cf2a9a4e071d4fd973db6a5a78c40f490667c@chromium.org>

coreos/chromite,coreos/chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite