TST: Set MPLBACKEND=Agg on test invocations

We do not need to use a gui method of plotting for test runs, and other
backends may fail on CI.

conftest.py

@@ -1,4 +1,5 @@
 """Setup common stuff for pytests."""
+
 import os
 
 import pytest
@@ -65,6 +66,15 @@ def pytest_addoption(parser):
     )
 
 
+def pytest_configure(config):
+    # Set the Agg backend for all test runs. This should be removed once
+    # xtgeo.plot is removed. This is to ensure that CI runs do not fail
+    # on Windows and macOS.
+    import matplotlib as mpl
+
+    mpl.use("Agg")
+
+
 @pytest.fixture(name="generate_plot")
 def fixture_generate_plot(request):
     if "ROXENV" in os.environ: