Improve test settings page instructions

[keevan]

https://github.com/catalyst/moodle-auth_saml2/blob/26b064c/classes/auth.php#L1166-L1178

Currently, if there are any conditions which prevent testing from happening, it returns a generic message as shown below.

In order to use this test, plugin needs to be configured, enabled and debugging mode should be enabled in plugin settings.

Instead, it would be much better for the user, if either:

• only those requirements, that require action were listed.
• OR all requirements were listed, but the ones that were completed / valid are displayed with a "Check" symbol (anything to indicate completed)