This is a testing bug. The HS17 b15 VM is part of 6u21 and is being tested as a 6u21 VM, but it actually got dropped into a 6u20 JDK.
HS_17-b15 PIT results for jdk6u21b05: 2010-05-21
JDK: Java(TM) SE Runtime Environment (build 1.6.0_20-b02)
VM: Java HotSpot(TM) Client VM (build 17.0-b15-201005202359.et151817.hs17b15, mixed mode, sharing)
If you run the test against the promoted 6u21b05 binaries it passes fine.
The fix for 6801020, for which the failing test is the regression test case, is only in 6u21 (and a bunch of revs) and not 6u20.
I believe this is due to a JPRT limitation, because 6u21 is not yet supported there. But it is a concern that we're testing the VM in the wrong JDK.