use 'test.root' property instead of traversing test-src path

several classes in vmTestbase need to determine the root directory of the test suite, currently, they do it by traversing the test-src path, http://hg.openjdk.java.net/code-tools/jtreg/rev/9d5ac9297316 added 'test.root' to jtreg set set of properties, so all these classes can be updated to use this property.