reflect: add test for invalid conversion

Conversion between slices with different element types is not allowed.
Previously (1.8 <= goversion <= 1.16), this conversion was allowed
if the base types were from different packages and had identical names.

Update #47785

Change-Id: I359de5b6fe3ff35bdbf9ab5a13902a0f820cac66
Reviewed-on: https://go-review.googlesource.com/c/go/+/343329


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>