Code coverage statistics for OpenJDK

[Martijn Verburg]

Hi all,

Recently, one of our Adopt OpenJDK incubator projects (for reference, these
projects are outside of OpenJDK proper, the wiki holds the full list)
managed to get some what we think are accurate code coverage stats for
OpenJDK (jdk9-dev) tests. Thanks to Jonathan Gibbons from code-tools
(jtreg/jcov) and Adopt's John Oliver for getting this out!

I think this is potentially useful for the OpenJDK quality group to report
alongside the existing weekly tests passing that Balchandra kindly pushes
out.  It can also be useful to OpenJDK contributors to have a guide on
making a change -> writing a test -> seeing code coverage improve.

Obviously we want to:

* Make sure the numbers are correct.

* Make it clear in the report that this does not represent how well OpenJDK
/ Java is actually tested (internally Oracle and others run a far more
comprehensive test suite).

Do people feel this is this a good idea? If so, who's the right
person/people to analyse our results and ensure we're using jtreg and jcov
correctly?

Cheers,
Martijn