Add new `TestInfo` value to snippet meta data

[delphidabbler]

Several of the "tests" in the tests directory are actually demos. Snippets demonstrated by these tests do not have TestInfo=advanced, which makes sense.

How about introducing a new demo value for TestInfo that can be applied to snippets that are demonstrated as part of a demo program.

[delphidabbler]

Use new TestURL property to link to the demo project or code.

[delphidabbler]

Didn't add new value to TestInfo because that would break backwards compatibility. Instead add new AdvancedTest.Level key to contain extended testing info and used AdvancedTest.URL instead of TestURL.

[delphidabbler]

Implemented by merge commit 22bef01