Add [test] section for /data/[nativetest|benchmarktest]

We have both system and vendor tests in the same directory
/data/nativetest. Since we can't distinguish system and vendor tests at
runtime, we choose to run all of them with the legacy namespace
configuration where /system/lib and /vendor/lib are both accessible
without any restriction. Furthermore, /system/lib/vndk-sp is added to
the list of accessible directories for tests since some libs (such as
libion.so) exist only in the directory but not in /system/lib.

This will change in the future when we install system and vendor tests
into different directories (vendor tests goes into ./vendor subdir).
Then, we will run the tests with [system] or [vendor] configurations
depending on their paths.

Bug: 63597267
Test: run bionic-unit-tests
Test: run linker-unit-tests
Change-Id: I810003b2da0b30335200c130f484ea7b041f9f80