Initial version of 'dx cp' regression test

Summary: minor change

Test Plan: This is a regression test for 'dx cp'

Reviewers: psung

Subscribers: akislyuk

Differential Revision: https://phabricator.dnanexus.com/D137
Merge branch 'dx-cp-regression-test-initial' of github.com:dnanexus/dx-toolkit into dx-cp-regression-test-initial
Merged the tests into test_dxclient.py. Used the assertSubprocessFailure function to test cases where 'dx cp' fails (and should fail).