RFR: 8273544: Increase test coverage for snippets

[Pavel Rappo]

This change is mostly about tests. I say "mostly" because while increasing code coverage, which was the primary goal of this exercise, I uncovered a few non-critical bugs and fixed them in-place. The net effect of the change boils down to these code coverage statistics.

before
------

%method	%block	%branch	%line
jdk.javadoc.internal.doclets.toolkit.taglets.SnippetTaglet	
75%(12/16)
87%(109/124)
88%(99/112)
85%(140/164)

#classes	%method	%block	%branch	%line
jdk.javadoc.internal.doclets.toolkit.taglets.snippet	
70%(80/114)
76%(310/407)
65%(178/273)
81%(413/508)

after
-----

%method	%block	%branch	%line
jdk.javadoc.internal.doclets.toolkit.taglets.SnippetTaglet	
100%(17/17)
95%(120/126)
93%(103/110)
97%(163/168)

%method	%block	%branch	%line
jdk.javadoc.internal.doclets.toolkit.taglets.snippet	
83%(94/112)
85%(348/405)
73%(202/273)
91%(463/505)

-------------

Commit messages:
 - Remove trailing whitespace for jcheck
 - Add negative tests for next-line markup
 - Use the feature from JDK-8273154
 - Test markup issues in an external snippet
 - Add TODO entries for FileManager-related issues
 - Improve error handling in SnippetTaglet
 - Cover blank and valueless "region" attribute
 - Add tests that showcases */ and \uxxxx
 - Encapsulate HTML representation of a snippet
 - Apply test a naming convention
 - ... and 6 more: https://git.openjdk.java.net/jdk/compare/2ca4ff87...3975b0c0

Changes: https://git.openjdk.java.net/jdk/pull/6359/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6359&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273544
  Stats: 1413 lines in 12 files changed: 1127 ins; 175 del; 111 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6359.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6359/head:pull/6359

PR: https://git.openjdk.java.net/jdk/pull/6359