Add more tests for EmPy template expansion

The 'positive' case was previously completely uncovered on Windows, and
there was also no test asserting that the caching mechanism respected
differing input variables.

test/test_shell_template.py

@@ -42,6 +42,17 @@ def test_expand_template():
  f" processing template '{template_path}'")
  assert not destination_path.exists()
 
+ # proper use
+ expand_template(template_path, destination_path, {'var': 'value1'})
+ assert destination_path.exists()
+ assert destination_path.read_text() == 'value1'
+ # overwrite with a different value
+ expand_template(template_path, destination_path, {'var': 'value2'})
+ assert destination_path.exists()
+ assert destination_path.read_text() == 'value2'
+
+ destination_path.unlink()
+
  # skip all symlink tests on Windows for now
  if sys.platform == 'win32': # pragma: no cover
  return