testrunner: Don't override env defaults

Allow customizing the different binaries used by testrunner, for example

DX=some-other-tool testrunner.py

will now use a custom DX. Previously it was hardcoded to always use
DX from HOST_OUT_INTERMEDIATES/dx.

Test: JACK=echo art/test/testrunner.py
Change-Id: I76133493767746561b44ffc69c5ef399bd8f9941