runtime: update TestExitHooks error strings

runtime: update TestExitHooks error strings

The error strings were changed in CL 586259 and no longer include
"internal error". Update the strings in the test.

Should fix the longtest builders.

Change-Id: If227d4b79a8bf5e52a32c3b2eceb086241f079aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/587757
Auto-Submit: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>