dashboard: unmark darwin-arm64 as missing a TryBot
dashboard: unmark darwin-arm64 as missing a TryBot

Issue 39782 is closed because we have darwin-arm64 builders by now.
A misc-compile TryBot currently provides pre-submit test coverage
for the darwin-arm64 port. So remove "darwin-arm64" known missing
entry since it's obsolete.

Also improve the test to report a problem when a port is marked as
having a known missing TryBot, yet it's not actually missing. This
should help humans not forget to keep the knownMissing map updated.

Change-Id: Ic5c981a81c2e6016a56c6d872db458c98f215a0e
Reviewed-on: https://go-review.googlesource.com/c/build/+/313209
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Carlos Amedee <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/build,golang/build,golang/build,golang/build