runtime: improve test file naming

The runtime builtin functions that are tested in append_test.go
are defined in slice.go. Renaming the test file to slice_test.go
makes this relation explicit with a common file name prefix.

Change-Id: I2f89ec23a6077fe6b80d2161efc760df828c8cd4
Reviewed-on: https://go-review.googlesource.com/90655
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>