Test resolving DOI

This helps distinguish temporary(?) errors in the DOI resolution from errors in the content provider

tests/unit/contentproviders/test_doi.py

@@ -30,8 +30,14 @@ def test_url_headers(requests_mock):
     assert result.request.headers["User-Agent"] == f"repo2docker {__version__}"
 
 
-def test_unresolving_doi():
+@pytest.mark.parametrize(
+    "requested_doi, expected",
+    [
+        ("10.5281/zenodo.3242074", "https://zenodo.org/records/3242074"),
+        # Unresolving DOI:
+        ("10.1/1234", "10.1/1234"),
+    ],
+)
+def test_doi2url(requested_doi, expected):
     doi = DoiProvider()
-
-    fakedoi = "10.1/1234"
-    assert doi.doi2url(fakedoi) is fakedoi
+    assert doi.doi2url(requested_doi) == expected