An issue with hotspot regression tests has been reported on openjdk6 mailing list:
http://mail.openjdk.java.net/pipermail/jdk6-dev/2017-February/003626.html

After updating Ubuntu to 6b41 I noticed that the new jtreg hotspot
testcase 7116786 was failing with "Unsupported major.minor version
51.0". This is due to the classes under
hotspot/test/runtime/7116786/testcases.jar being all compiled for Java
7.

The testcase was added by:
http://hg.openjdk.java.net/jdk6/jdk6/hotspot/rev/6088554a147c

Download testcases.jar directly from the repo @
http://hg.openjdk.java.net/jdk6/jdk6/hotspot/file/tip/test/runtime/7116786/testcases.jar

As expected, this also affects the 6u41 tarball @
https://java.net/downloads/openjdk6/openjdk-6-src-b41-04_jan_2017.tar.xz

Running 'file' on the unpacked jar file:
$ file *.class
case00.class: compiled Java class data, version 51.0 (Java 1.7)
case01.class: compiled Java class data, version 51.0 (Java 1.7)
case02.class: compiled Java class data, version 51.0 (Java 1.7)
case03.class: compiled Java class data, version 51.0 (Java 1.7)
case04.class: compiled Java class data, version 51.0 (Java 1.7)
case05.class: compiled Java class data, version 51.0 (Java 1.7)