simple_test_case

It's used only as a dev dependency in 5 crates.

Depender (with downloads and own rev deps)			simple_test_case version
270	3	penrose	dev	^1
120		simple_coro	dev	^1
	2	ad_event	dev	^1
		ad-editor	dev	^1.2.0
	2	ninep	dev	^1