if an "absolute" path for a testlibrary used in lib.dirs, jtreg won't add a library from lib.dirs if it's located in external root (external.lib.roots), because it c.s.j.r.config.TestProperties does not (and most probably should not) use external roots to locate a corresponding file in initLibDirSet method.

Using a relative path is also impossible, because for such paths 'rootDir.toURI().relativize(f.toURI())).toString()' contains a scheme name - "file:", so libDirs set will contain a buggy library path, e.g. /file:/Users/iignatye/ws/jdk10/jdk10/test/lib/. fixing this problem by using toPath instead of toURI, makes jtreg to report an expected error --
"effective library path is outside the test suite"