Minimize the classpath pants passes to java for running tests.

This change decreases the size of the classpath pants passes to java for
running tests. It is necessary because this classpath can grow very large, and
exceed the maximum argument length on some environments.

Really the best place for this change was in
executor.py:Executor:_create_command
But the proper solution would involve changing self._command to a function
that returned a context manager (since a temporary jar would need to be
created to be used for the new classpath) and this would require refactoring a
considerable amount of code.
This fix is also a bandaid fix, as it does not ultimately solve the classpath
length problem (it only removes jars from the classpath).
Because of the fact that this fix is hopefully temporary, and a refactor now
would require another refactor when a better solution is implemented, I
instead opted to make the classpath change in the client code rather than in
the lower level java executor code.

I did not add tests/logging because I did not find any existing tests/logging
in the source (it would also probably require heavy mocking). If you think the
code needs some please let me know and I'll add it.

By Dan Davydov at https://rbcommons.com/s/twitter/r/205/