RFR: 8276124: Provide snippet support for properties files [v3]

[Pavel Rappo]

> The initial integration (JDK-8266666) of JEP 413 did not support properties files. This commit rights that wrong.

Pavel Rappo has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 22 commits:

 - Merge branch 'master' into 8276124
 - Merge branch '8273544' into 8276124
   
   Bring changes from the originating branch, added after this branch was created.
 - Repair a confusing commit 93f01b9
 - Address feedback
   
    - Hoist the tb (toolbox) field to SnippetTester
    - Use use default ctors instead of private no-arg ctors for test classes
    - Re-implement SnippetTester.checkOutputEither on top of JavadocTester.OutputChecker.checkAnyOf
    - Follow convention for jtreg comment
    - Improve method names with underscores
 - Initial commit
 - Test one more corner case example
 - Remove trailing whitespace for jcheck
 - Add negative tests for next-line markup
   
   Covers extra 2 lines of MarkupParser: [101, 102]
 - Use the feature from JDK-8273154
 - Test markup issues in an external snippet
   
   Covers extra 5 lines of SnippetTaglet: [244, 248]
 - ... and 12 more: https://git.openjdk.java.net/jdk/compare/2ab43ec2...bd9aadab

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Changes: https://git.openjdk.java.net/jdk/pull/6397/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=02
  Stats: 332 lines in 5 files changed: 290 ins; 26 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6397/head:pull/6397

PR: https://git.openjdk.java.net/jdk/pull/6397