build_support: always clean out build_root/test when action=test
Some old results (e.g. a llvmpipe generated test failure) might stick around so this makes sure the test dir in the build_root is removed at the start of testing.
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Some old results (e.g. a llvmpipe generated test failure) might stick around so this makes sure the test dir in the build_root is removed at the start of testing.