Build: Avoid using ppid to generate temporary directory name.

When TMPDIR is left unset, use the BSD checksum of
$(ANDROID_BUILD_TOP)-$(USER) instead of the PPID to construct a root
directory for all test runs. This workaround lets us run tests from
multiple repos at the same time, and will also avoid having to deal
with path length issues on the build bots.

This is a hacky workaround, but the only proper fix is stop using a
test runner built on top of make.

Bug: 32165422
Test: test-art-host

Change-Id: I61bc1eb3cf57d498f910573926d826a36020067a