Only extract zipapex once for linux-bionic-zipapex tests

We were unzipping the zipapex for every test when running the
art-linux-bionic-x64-zipapex tests. This could take a long time. This
makes us only have to unzip it a single time.

Test: ./test/testrunner/run_build_test_target.py art-linux-bionic-x64-zipapex
Change-Id: Id7cf1b6445526a950c45b74fc98208e234d3069f
4 files changed