write tests to validate snippets in java.compiler and jdk.javadoc API

One of the goals of providing snippets was to make it possible or at least easier to write tests to validate snippets, using existing test frameworks (such as jtreg), possibly assisted by library code to assist with common operations, such as locating snippets by `element` and `id`, and parsing or compiling snippets.

We should demonstrate the ability by providing tests for recently added new snippets.