Remove `allow(clippy::if_then_panic)` from spread allocate tests

Follow up to #995.

As per the code: 
> The `synstructure` crate currently expands to code that has a single
> `panic` inside an `if` block. We have to allow the following `clippy`
> lint until the fixing PR has been merged:
> PR: https://github.com/mystor/synstructure/pull/50
