As noted in #12843, we’re currently missing bugs unique to bitcoin-qt during the Travis build because the functional test framework only uses bitcoind (and not bitcoin-qt).
Run a single Travis job under xvfb for framebuffer virtualization and using bitcoin-qt as the binary. I’ve also removed the NEED_XVFB
configuration since our usage of xvfb is outdated per Travis’ docs and the corresponding pre-build step is no longer necessary.