bytes: improve consistency in split test messages

bytes: improve consistency in split test messages

This change improves consistency in the error messages used by TestSplit and TestSplitAfter in bytes_test.go

Change-Id: Idc4e77a516037ce17cbb1bdffe3ac00bcde5de42
GitHub-Last-Rev: 8c4a35186851004251e8eee71c88ca1c822851c1
GitHub-Pull-Request: golang/go#76970
Reviewed-on: https://go-review.googlesource.com/c/go/+/732300
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jorropo <jorropo.pgm@gmail.com>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>