bytes: merge explodetests into splittests

splittests already contains most of the tests that cover explode. Add
the missing ones and skip the append test for empty results which would
otherwise lead to an "index out of range" panic.

Change-Id: I2cb922282d2676be9ef85f186513075ae17c0243
Reviewed-on: https://go-review.googlesource.com/c/go/+/170126


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>