Make Rust test harness optional for test binaries

Adds a test_harness field to Rust test blueprint properties. If this
field is explicitly set to false, do not build the test binary with
`--test`. This is necessary for Rust tests which cannot use the standard
test harness.

Test: atest rustBinderTest
Test: atest --host proc-macro2_tests_test
Change-Id: Icbcb54422cc716348feae56b2d71f013516b0ac0