Tests on FP Math.abs()

Rationale:
As a "quality of implementation" rather than pure "spec compliance"
we require that Math.abs() clears the sign bit (but changes nothing
else) for all numbers, including NaN.

These tests ensure that remains the case.

**NOTE**
This CL depends on the pending libcore change
(https://android-review.googlesource.com/#/c/316368/).

Test: test-art-host
Bug: 30758343

Change-Id: I01044a73992b2e89d97060ed8bbb76abea0933b9