test_environment: improve error reporting

Author: mr-c

tests/test_environment.py

@@ -297,7 +297,9 @@ def test_preserve_all(
     for vname, val in env.items():
         try:
             assert_envvar_matches(checks[vname], vname, env)
-        except KeyError:
+        except KeyError as kerr:
+            if vname not in os.environ:
+                raise kerr
             assert val == os.environ[vname]
         except AssertionError:
             if vname == "HOME" or vname == "TMPDIR":