RFR: 8251888: Move HotSpot Style Guide wiki subpages to jdk/jdk/doc

[Kim Barrett]

Please review this followup to JDK-8247976 [1], rehoming the information
from two subpages of the wiki Style Guide to jdk/jdk/doc.

The "Naming HotSpot JTReg Tests" wiki page has been summarized as an
additional small section in the HotSpot Style Guide.

The "Native/Unit Test Development Guidelines" has been transferred wholesale
to a new file: jdk/jdk/doc/hotspot-unit-tests.{md,html}. This transfer was
accomplished by cut-and-paste of the wiki page content to the new file, then
adding markdown annotations corresponding to the wiki page's formatting.  No
significant modifications were made to the text, not even to correct typos
and other errors that seem obvious.  I want to get the file in place before
doing any updates to the content.

[1] https://bugs.openjdk.java.net/browse/JDK-8247976 

I think this change doesn't modify the Style Guide in any substantive way,
so I think a simple review is sufficient, rather than needing to gather
consensus for the change from the Group Members.

Once this change is pushed I'll update the wiki pages to note they are
obsolete and provide a forwarding pointer.

CR:
https://bugs.openjdk.java.net/browse/JDK-8251888

Webrev:
https://cr.openjdk.java.net/~kbarrett/8251888/open.00/