Issue #3487: turned on checkstyle cache for developers

[rnveach]

Issue #3487

Cache turned on for Checkstyle and sevntu.

• Changes in XDocPagesTest is because instantiating the examples caused our own cachefile to be corrupted.
• Sevntu cachefile couldn't be enabled in the pom, that I could figure out. <cacheFile>${project.build.directory}/cachefile-sevntu</cacheFile> caused no cache file to be created.
• Removed the delete cachefiles as we don't really need them and will defeat the purpose of the cache. CI will always be clean, so it will only matter locally. Cache is deleted on clean, and will reset for files/configs that change. Deleting the file will just put us back without cache each time we run verify.

[codecov-io]

Current coverage is 99.59% (diff: 100%)

Merging #3565 into master will not change coverage

@@             master      #3565   diff @@
==========================================
  Files           275        275          
  Lines         13311      13311          
  Methods           0          0          
  Messages          0          0          
  Branches       3035       3035          
==========================================
  Hits          13257      13257          
  Misses           53         53          
  Partials          1          1          

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[romani]

Sevntu cachefile couldn't be enabled in the pom, that I could figure out

issue created against maven by you for cache: https://issues.apache.org/jira/browse/MCHECKSTYLE-332