Fix class loader issues in instrumentation tests.
Fix class loader issues in instrumentation tests.

The ASM ClassWriter uses a wimpy class loader when computing common
superclasses. This could cause a ClassNotFoundException in the
transform method (at reader.accept). This exception gets swallowed,
resulting in a class that should be instrumented to silently not
be. The fix is to override getCommonSuperClass to use the correct
class loader.

Trivia: This bug was discovered while 'stress-testing' this
instrumentation scheme on the Coursera students, to check that they
implement one method in terms of another in the assignment.

som-snytt/scala-partest,SethTisue/scala-partest,lrytz/scala-partest,scala/scala-partest,scala/scala-partest