See JT CR 6633810
A user complained that execution of particular tests takes a lot of time unlike older versions of JTReg/JavaTest. Actually running tests is the same, report generation is differ - current generates report for all test suite iterating over all the tests.

Solution like ( this is in Report.Settings::getInitialFiles() )

if ("regtest".equals(ip.getTag())) { // special case for JTReg
  String[] s = ip.getTests();

  if (s != null) {
     File[] initFiles = new File[s.length];
     for (int i = 0; i< s.length; i++) initFiles[i] = new File(s[i]);

     return initFiles;
  }
}

works but it's ugly to hardcode "regtest" in JT. Unfortunately I don't see any good
solution changing only JT and not touching JTReg.