Enable xvfb in travis to allow running test_bitcoin-qt

Avoids following error:

QXcbConnection: Could not connect to display

.travis.yml

@@ -32,7 +32,7 @@ env:
 # bitcoind
     - HOST=x86_64-unknown-linux-gnu PACKAGES="bc python3-zmq" DEP_OPTS="NO_QT=1 NO_UPNP=1 DEBUG=1" RUN_TESTS=true GOAL="install" BITCOIN_CONFIG="--enable-zmq --enable-glibc-back-compat --enable-reduce-exports CPPFLAGS=-DDEBUG_LOCKORDER"
 # No wallet
-    - HOST=x86_64-unknown-linux-gnu PACKAGES="python3" DEP_OPTS="NO_WALLET=1" RUN_TESTS=true GOAL="install" BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports"
+    - HOST=x86_64-unknown-linux-gnu PACKAGES="python3 xvfb" DEP_OPTS="NO_WALLET=1" RUN_TESTS=true GOAL="install" BITCOIN_CONFIG="--enable-glibc-back-compat --enable-reduce-exports"
 # Cross-Mac
     - HOST=x86_64-apple-darwin11 PACKAGES="cmake imagemagick libcap-dev librsvg2-bin libz-dev libbz2-dev libtiff-tools python-dev" BITCOIN_CONFIG="--enable-gui --enable-reduce-exports" OSX_SDK=10.11 GOAL="deploy"
 
@@ -50,6 +50,8 @@ before_script:
     - if [ -n "$OSX_SDK" -a ! -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then curl --location --fail $SDK_URL/MacOSX${OSX_SDK}.sdk.tar.gz -o depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi
     - if [ -n "$OSX_SDK" -a -f depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz ]; then tar -C depends/SDKs -xf depends/sdk-sources/MacOSX${OSX_SDK}.sdk.tar.gz; fi
     - make $MAKEJOBS -C depends HOST=$HOST $DEP_OPTS
+    # Start xvfb if needed, as documented at https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-a-GUI
+    - if [ "$RUN_TESTS" = "true" -a "${DEP_OPTS#*NO_QT=1}" = "$DEP_OPTS" ]; then export DISPLAY=:99.0; /sbin/start-stop-daemon --start --pidfile /tmp/custom_xvfb_99.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :99 -ac; fi
 script:
     - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then while read LINE; do travis_retry gpg --keyserver hkp://pool.sks-keyservers.net --recv-keys $LINE; done < contrib/verify-commits/trusted-keys; fi
     - if [ "$CHECK_DOC" = 1 -a "$TRAVIS_REPO_SLUG" = "bitcoin/bitcoin" -a "$TRAVIS_PULL_REQUEST" = "false" ]; then git fetch --unshallow; fi