Enable GUI tests in VSTS Linux builds via xvfb-run (GH-7890)

diff --git a/.vsts/linux-deps.yml b/.vsts/linux-deps.yml
index b6c8a36..83b0b59 100644
--- a/.vsts/linux-deps.yml
+++ b/.vsts/linux-deps.yml
@@ -31,6 +31,7 @@
     liblzma-dev
     libffi-dev
     uuid-dev
+    xvfb
   displayName: 'Install dependencies'
 - script: python3 Tools/ssl/multissltests.py --steps=library --base-directory $(build.sourcesDirectory)/multissl --openssl $(OPENSSL) --system Linux
   displayName: 'python multissltests.py'