Skip GUI tests if xvfb not installed

Author: mfisher87

Makefile

@@ -1,7 +1,7 @@
 .PHONY: test
 test:
 	python -m pytest --version
-	python -m pytest --xvfb-backend=xvfb -v test/
+	python -m pytest -v test/
 
 
 .PHONY: lint 

doc/contributing.md

@@ -55,8 +55,7 @@ make test
 ```
 
 Unit tests require `xvfb` (X Virtual Framebuffer) to test the GUI. If `xvfb` is not
-installed, you'll receive `ERROR: xvfb backend xvfb requested but not installed`.
-Install with:
+installed, GUI tests will be skipped. Install with:
 
 ```bash
 sudo apt install xvfb

test/test_gui.py

@@ -1,8 +1,17 @@
 from pathlib import Path
 
+import pytest
+import pytest_xvfb
+
 from viscm.cli import _make_window
 
+xvfb_installed = pytest_xvfb.xvfb_instance is not None
+
 
+@pytest.mark.skipif(
+    not xvfb_installed,
+    reason="Xvfb must be installed for this test.",
+)
 class TestGui:
     def test_gui_view_opens(self, qtbot):
         window = _make_window(