syspath: correctly prefix Windows UNC paths

Identified while tackling #670, but this should actually solve some legitimate
problems with cataloging music on a network drive.
skip pyechonest tests when library not installed
more test isolation (#670)

Doing test-specific cleanup in tearDown before general sandbox deletion helps
avoiding contamination of global state between tests when cleanup fails.

Current Windows status:

    Ran 1106 tests in 72.373s
    FAILED (SKIP=10, errors=13, failures=15)

Closer!