fix CSharpTemp.TestDestructors

TestDestructors test cannot be run twice as a static variable holds data from
the first run.

Add an init function to set the static variable to a known state.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@iki.fi>
Merge pull request #257 from tomba/fix-test

fix CSharpTemp.TestDestructors