Change `unittest.skipIf` decorator into an assertion for some tests

[alanking]

• test_temporary_xml_mode_change_with_operation_as_proof__issue_586
• test_setting_xml_parser_choice_by_environment_only__issue_584

From these review comments: #589 (comment) and #585 (comment)

These tests will be skipped if the environment in which the test is running is not found to be in the expected state. This could lead to the test being skipped unintentionally. If we want the test to always be run, we should assert that the environment has the expected conditions rather than skipping the test. Skipping the test could lead us to believe that all our tests are passing when in fact they are not, just because the testing environment is not in the expected state.