Make compiler-related gtests honor ART_USE_OPTIMIZING_COMPILER.

Previously, gtests using the art::CommonCompilerTest
class were using Quick as compiler.  Now, setting the
environment variable ART_USE_OPTIMIZING_COMPILER to `true`
before building and running these tests will use Optimizing
instead.

Change-Id: I724a3215d2eb1841089745fbabb5cb58b0422ef3