Make sure each test finishes clean.
test: Source DataStream must be reachable during Result DataStream export

Addressing:
Error: test_session_load(TestSession)
  OpenSCAP::OpenSCAPError: Unable to open file: 'test/data/sds-complex.xml' [oscapxml.c:573]
lib/openscap/openscap.rb:32:in `raise!'
lib/openscap/xccdf/session.rb:20:in `initialize'
test/xccdf/session_test.rb:29:in `new'
test/xccdf/session_test.rb:29:in `test_session_load'
test: Remove working directory even if there is content

Addressing:
Errno::EEXIST: File exists - test/output
test/xccdf/session_test.rb:49:in `mkdir'
test/xccdf/session_test.rb:49:in `to_work_dir'
test/xccdf/session_test.rb:36:in `test_session_export'
Ignore tests output.