Widen Error Type to Throwable in ZIO Test Interop

[adamgfraser]

When we originally implemented the interop support for ZIO Test, the test methods required the error type to be Nothing, so we translated any Throwable in the F effect to a die failure using orDie. Subsequently we made the test methods in ZIO Test polymorphic in the error type, so now we can stop doing that and just propagate the potential Throwable failure through.

[neko-kai]

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