RFR: JDK-8278795: Create test library and tests for langtools snippets

[Jonathan Gibbons]

Please review a test-only fix to provide a new test library for analyzing snippets, and two tests that use that library to test snippets in the `jdk.javadoc` and `jdk.compiler` modules.

It is expected that the library may evolve in future updates as we explore additional ways to analyze snippets.

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Commit messages:
 - JDK-8278795: Create test library and tests for langtools snippets

Changes: https://git.openjdk.java.net/jdk/pull/6839/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6839&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278795
  Stats: 804 lines in 3 files changed: 804 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6839.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6839/head:pull/6839

PR: https://git.openjdk.java.net/jdk/pull/6839