env.py: Add per-lisa-test command results folder

The results directory used by tests currently defaults to a
timestamp-based folder name, created when the first TestEnv of the test
session is instantiated. Since they share the state, subsequent test
cases reuse the same folder.

Cleanup that behavior by introducing true per-lisa-test invocation
directory, and storing the tests results in folders named after the test
case class or after a timestamp. The timestamp naming is mainly intended
to be used from a notebook where a TestEnv is instantiated by hand.

The added benefits are that results.xml xUnit file has a permanent home,
and that the tests results folders are clearly named after the class
that created them.