tests: Older ruby's TestCase does not love to cleanup()

Now the test pass on Red Hat Enterprise Linux 6.

Addressing:
  1) Error:
test_double_read_error(TestOscapVersion):
Errno::ENOENT: No such file or directory - test/output
    ./test/common/testcase.rb:32:in `mkdir'
    ./test/common/testcase.rb:32:in `setup'