Improve hbase ref guide for the testing part.

Here is some stuff I saw. I will propose a fix in a week or so, please add the comment or issues you have in mind.

15.6.1. Apache HBase Modules
=> We should be able to use categories in all modules. The default should be small; but any test manipulating the time needs to be in a specific jvm (hence medium), so it's not always related to minicluster.

15.6.3.6. hbasetests.sh
=> We can remove this chapter, and the script
The script is not totally useless, but I think nobody actually uses it.

=> Add a chapter on flakiness.
Some tests are, unfortunately, flaky. While there number decreases, we still have some. Rules are:

• don't write flaky tests!
• small tests cannot be flaky, as it blocks other test execution. Corollary: if you have an issue with a small test, it's either your environment either a severe issue.
• rerun the test a few time to validate, check the ports and file descriptors used.

mvn test -P localTests -Dtest=MyTest
=> We could actually activate the localTests profile whenever -Dtest is used. If we do that, we can remove the reference from localTests in the doc.

mvn test -P runSmallTests mvn test -P runMediumTests
=> I'm not sure it's actually used. We could remove them from the pom.xml (and the doc).

The HBase build uses a patched version of the maven surefire plugin
=> Hopefully, we will be able to remove this soon

Integration tests are described TODO: POINTER_TO_INTEGRATION_TEST_SECTION
=> Should be documented