Fix tests: restore the monkey patch after testing Paths

To test path vars,  we monkey patched the method get_prefix of
Paths, but not restored.  Because of it, the other test cases, like
test_plugin, fail to pass.  This patch restores the change on teardown.

Reported-by: Zhou Zheng Sheng <zhshzhou@linux.vnet.ibm.com>
Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>