The "test root directory", or "test suite root directory" is the root directory of the overall test suite. In OpenJDK terms, this means either of the "jdk/test/" or "langtools/test/" directories in an OpenJDK forest.
The "test root directory" for any test is determined by finding the smallest enclosing directory containing a marker file called TEST.ROOT. The TEST.ROOT file can also be used to define some global properties for the test suite.