reflect: add test of maps with keys larger than key size
This finds the bug fixed in CL 630279.
reflect mutates the SwissMapType of a map[unsafe.Pointer]unsafe.Pointer,
which happened to already have the correct GroupSize for all of the maps
used in the reflect tests.
For #54766.
Change-Id: If4428e1e799598e7512edceb3cefb2ad00cfa712
Reviewed-on: https://go-review.googlesource.com/c/go/+/630676
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@golang.org> Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>