Add unit test for MRAN death

tests/unit/test_r.py

@@ -72,3 +72,14 @@ def test_snapshot_rspm_date(base_image):
 
     with pytest.raises(ValueError):
         r.get_rspm_snapshot_url(date(1691, 9, 5))
+
+
+def test_mran_dead(tmpdir, base_image):
+    tmpdir.chdir()
+
+    with open("runtime.txt", "w") as f:
+        f.write('r-3.6-2017-06-04')
+
+    r = buildpacks.RBuildPack(base_image)
+    with pytest.raises(RuntimeError, match=r'^Microsoft killed MRAN, the source of R package snapshots before 2018-12-07.*'):
+        r.get_build_scripts()