GoogleTest: Use googletest-generated JSON for test discovery

I noticed that unlike gtest_add_tests the newer command gtest_discover_tests doesn't provide the file and line where a certain test has been defined - and I think this is a nice feature, in particular if there are many tests. Google test provides this information through a xml-file, therefore, I changed the cmake-file that is responsible for the test discovery so that it parses this xml-file instead of the command line output of the google-test command.

UPDATED: The original description above focused on using --gtest_output=xml, but following discussion, we have switched to --gtest_output=json instead.

Topic-rename: gtest-use-json-output-for-discovery