RFR: 8273544: Increase test coverage for snippets [v4]

[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)

Pavel Rappo has updated the pull request incrementally with one additional commit since the last revision:

  Repair a confusing commit 93f01b9

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6359/files
  - new: https://git.openjdk.java.net/jdk/pull/6359/files/981cc305..b7c8c48a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6359&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6359&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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