Expand testing of necessary features for print-changed=dot-cfg.

Summary:
Expand the testing for whether the lit tests for print-changed=dot-cfg
are supported to include checking whether dot supports pdf output.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: hvdijk (Harald van Dijk)
Differential Revision: https://reviews.llvm.org/D113187