build: add work-around for JDK bug and PowerMock

When mocking recalcitrant classes, the unit-tests uses powermock
to generate and inject the necessary byte-code to allow testing
independent of the underlying platform.

With OpenJDK v1.7.0_60, powermock-generated byte-code functions
correctly.  With OpenJDK v1.7.0_65, this generated byte-code
fails will errors like:

    java.lang.VerifyError: Bad <init> method call from inside of a branch
    Exception Details:
      Location:
        org/dcache/gplazma/plugins/NsswitchTest$SimpleIntStorage.<init>(Lorg/dcache/gplazma/plugins/NsswitchTest;)V @39: invokespecial
      Reason:
        Error exists in the bytecode
      Bytecode:
        0000000: 2a2b b500 142a 4d12 16b8 001c 03bd 000f
        0000010: 121e b800 22b8 0028 3a04 1904 b200 2ca5
        0000020: 000e 2a01 c000 2eb7 0031 a700 092c b700
        0000030: 3301 57b1
      Stackmap Table:
        full_frame(@45,{UninitializedThis,Object[#13],UninitializedThis,Top,Object[#15]},{})
        full_frame(@51,{Object[#2],Object[#13],Object[#2],Top,Object[#15]},{})

            at sun.reflect.GeneratedSerializationConstructorAccessor7.newInstance(Unknown Source)
            at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
            at org.objenesis.instantiator.sun.SunReflectionFactoryInstantiator.newInstance(SunReflectionFactoryInstantiator.java:40)
            at org.powermock.reflect.internal.WhiteboxImpl.newInstance(WhiteboxImpl.java:229)
            at org.powermock.reflect.Whitebox.newInstance(Whitebox.java:139)
            at org.dcache.gplazma.plugins.NsswitchTest.setUp(NsswitchTest.java:86)

This problem has been reported and will hopefully be fixed at some
point, but meanwhile, this patch adds a work-around.

Target: master
Patch: https://rb.dcache.org/r/7185/
Acked-by: Gerd Behrmann
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6