Make InUnitTestRunnerOverride work as expected

Setting the InUnitTestRunnerOverride does not actually allow one to easily set the DeferredScheduler. The scheduler has to be cleared when still "in unit test runner" and then reset when the override is in place..

This is unexpected behavior, at least to me, so I fixed it :)

Sincerely
George

P.S.
I only came across this issue, because RxApp thought it was in the unit test runner, when in fact it was running on a WP8 emulator.
I don't know why, though.