Reduces garbage left behind in /tmp

o RRD file location when impermanent db was supplied as a directory, which
  made opening it always fail and create one extra "rrd-invalid"
  file/directory that the tests didn't clean up.
o Generally making sure tests that create temp file deletes them in @After
Merge pull request #790 from tinwelint/2.0-reduce-tmp-garbage

Reduces garbage left behind in /tmp after tests