Run tests with xvfb if and only if xvfb present
Run tests with xvfb if and only if xvfb present

This allows to run pipe tests on Linux (requires X) and with the same
makefile on OSX.

fabiorusso/chrome-token-signing,cristiano-andrade/chrome-token-signing,open-eid/chrome-token-signing,metsma/chrome-token-signing,cristiano-andrade/chrome-token-signing,open-eid/chrome-token-signing,metsma/chrome-token-signing,fabiorusso/chrome-token-signing,metsma/chrome-token-signing,open-eid/chrome-token-signing,fabiorusso/chrome-token-signing,cristiano-andrade/chrome-token-signing,open-eid/chrome-token-signing,fabiorusso/chrome-token-signing,cristiano-andrade/chrome-token-signing,metsma/chrome-token-signing,open-eid/chrome-token-signing,metsma/chrome-token-signing