Add some basic tests for easing functions

[scottmcm]

Objective

Context for #16676

The release notes nerd-sniped me again. I was looking at the EaseFunction::Exponential* definition and wondering why that specific definition was chosen. And looking into it I noticed that it seemed wrong and there weren't any tests, so here are those tests -- which are failing as proof.

Solution

I don't know what the best answer is here, but I think having tests is at least a good start for a discussion.

Testing

These are tests 🙂

[scottmcm]

The tests fail:

---- curve::easing::tests::ease_functions_zero_to_one stdout ----
thread 'curve::easing::tests::ease_functions_zero_to_one' panicked at crates/bevy_math/src/curve/easing.rs:490:13:
EaseFunction.ExponentialIn(0) was 0.0009765625

see the mentioned issue for discussion.