Exclude scala-xml from transitive dependencies of partest.

As discovered here:
https://github.com/scala/scala-xml/pull/6#issuecomment-26614894
we would have two different versions of scala-xml on test classpath:

  1. scala-xml classes compiled from source (we are in scala-xml
     project)
  2. scala-xml transitive dependencies of partest

Duplicated classpath entries are always dangerous. I implemented a
set of exclude rules that get rid of transitive dependencies of
scala-xml. This fixes the problem of JUnit using wrong version of
scala-xml for testing (now there's only one choice).

Also, this allowed me to to enable conflictWarning again. They are
very useful to warn us against dangerous situations that almost always
result in broken classpath (as we just learned).
Migrate a few partest jvm tests to JUnit tests.

Migrated to JUnit the following tests (from test/files/jvm):

   * t0632.scala
   * t1118.scala
   * unittest_xml.scala

None of those tests were dependent on anything jvm specific.
Remove test/files/jvm/backendBugUnapply.scala test.

It doesn't seem to test anything XML-specific and I couldn't trace
what kind of backend problem this test is supposed to guard against.
Migrate partest jvm/xmlstuff.scala test to JUnit.
Migrate partest jvm/xmlpull.scala test to JUnit.
Migrate partest jvm/xmlattr.scala test to JUnit.
Migrate partest jvm/xml04embed.scala test to JUnit.
Migrate partest jvm/xml03syntax.scala test to JUnit.
Merge pull request #6 from gkossakowski/master

Migrate majority of partest jvm tests to JUnit