Rollup merge of #82233 - ijackson:try-block-type-test, r=Mark-Simulacrum

Rollup merge of #82233 - ijackson:try-block-type-test, r=Mark-Simulacrum

try-back-block-type test: Use TryFromSliceError for From test

Using `i32` is rather fragile because it has many implementations.  Recently in an early draft of another MR (#82228) I did something that introduced a new `i32 as From<something>` impl and this test broke.

TryFromSliceError is nice because it doesn't seem likely to grow new conversions.  We still have one conversion, from Infallible.

My other MR is going to be reworked and won't need this any more but having done it I thought I would submit it rather than just throw it away.  Sorry for the tiny MR.