test: Skip desugar with DESUGAR=false when running tests

Allow one to skip desugar when running the tests, for example:

   $> ANDROID_COMPILE_WITH_JACK=false DESUGAR=false DX=<your-dx-tool> art/test.py

Why this might be useful: If building with javac and a dexer tool that
already processes invoke-dynamic lambdas, then desugar is redundant.

Bug: 36902714
Change-Id: I39aadca4985e2aafe4b4b24ff3dc9d7a364bbad7