RFR [jdk11]: 8232880 : Update test documentation with additional settings for client UI tooltip tests

[Baesken, Matthias]

Hello, please review  the following downport  of  8232880  to jdk11 .

The doc/testing.md  adjustment of the jdk/jdk  change  applies cleanly, and as expected the html needs to be regenerated from it with "make update-build-docs" .


https://bugs.openjdk.java.net/browse/JDK-8232880

http://cr.openjdk.java.net/~mbaesken/webrevs/8232880_jdk11/

Thanks, Matthias