iotests: add testing shim for script-style python tests

iotests: add testing shim for script-style python tests

Because the new-style python tests don't use the iotests.main() test
launcher, we don't turn on the debugger logging for these scripts
when invoked via ./check -d.

Refactor the launcher shim into new and old style shims so that they
share environmental configuration.

Two cleanup notes: debug was not actually used as a global, and there
was no reason to create a class in an inner scope just to achieve
default variables; we can simply create an instance of the runner with
the values we want instead.

Signed-off-by: John Snow <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Message-id: 20190709232550[email protected]
Signed-off-by: John Snow <[email protected]>