"package root" refers the root of the package hierarchy in which a Java source is placed. It is the directory you would put on the javac source path if you wanted javac to compile the file explicitly.
Most jtreg tests are written in the "unnamed package" (i.e. without a package statement) and so for most jtreg tests, the directory directly containing the test is the package root for the test. This is different from other test suites using other test harnesses (such as TestNG) in which all the tests of a test suite are placed in a single package hierarchy, often mirroring the package hierarchy of API being tested.