Use Xvfb on Linux build servers

Creates an X server for unit tests and user documentation generation and
tests.

buildconfig/Jenkins/buildscript

@@ -15,11 +15,14 @@ SCRIPT_DIR=$(dirname "$0")
 # System discovery
 ###############################################################################
 USE_CORE_DUMPS=true
+X11_RUNNER=""
 if [[ ${NODE_LABELS} == *rhel7* ]] || [[ ${NODE_LABELS} == *centos7* ]] || [[ ${NODE_LABELS} == *scilin7* ]]; then
   ON_RHEL7=true
+  X11_RUNNER="xvfb-run"
 fi
 if [[ ${NODE_LABELS} == *ubuntu* ]]; then
   ON_UBUNTU=true
+  X11_RUNNER="xvfb-run"
 fi
 if [[ ${NODE_LABELS} == *osx* ]]; then
   ON_MACOS=true
@@ -347,7 +350,7 @@ userprops_file=$userconfig_dir/Mantid.user.properties
 echo MultiThreaded.MaxCores=2 > $userprops_file
 
 if [[ ${DO_UNITTESTS} == true ]]; then
-  $CTEST_EXE -j${BUILD_THREADS:?} --schedule-random --output-on-failure
+  ${X11_RUNNER} $CTEST_EXE -j${BUILD_THREADS:?} --schedule-random --output-on-failure
 fi
 
 ###############################################################################
@@ -362,8 +365,8 @@ if [[ ${DO_DOCTESTS_USER} == true ]]; then
     rm -fr $BUILD_DIR/docs/doctrees/*
   fi
   # Build HTML to verify that no referencing errors have crept in.
-  ${CMAKE_EXE} --build . --target docs-html
-  ${CMAKE_EXE} --build . --target docs-test
+  ${X11_RUNNER} ${CMAKE_EXE} --build . --target docs-html
+  ${X11_RUNNER} ${CMAKE_EXE} --build . --target docs-test
 fi
 
 ###############################################################################