reflect: test GC bits for slices

reflect: test GC bits for slices

Currently we test bitmap repetitions constructed by the compiler (for
small arrays) and repetitions performed by GC programs (large arrays
and reflect-constructed arrays), but we don't test dynamic repetitions
performed by the runtime for slice backing stores. Add tests that
parallel the array tests using slices.

Change-Id: If4425941a33cc5b20178dd819a7371e347e47585
Reviewed-on: https://go-review.googlesource.com/11422
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>