dashboard: consider disabled TryBot as not providing test coverage
dashboard: consider disabled TryBot as not providing test coverage

Some TryBot builders are disabled or configured with a custom
try policy that doesn't test the master branch of the main Go
repository. For example, openbsd-386-68 is configured with:

	tryBot: explicitTrySet("sys"),

So it's only a TryBot for the x/sys repo, not the main one.

Modify TestTryBotsCompileAllPorts to not consider such TryBots
when determining whether a port has sufficient pre-submit test
coverage.

This will help in the next change which re-arranges target
selection between misc-compile TryBots.

For golang/go#32632.

Change-Id: If393610d11b04b75133a4f10f89cfdfff9dbaece
Reviewed-on: https://go-review.googlesource.com/c/build/+/313533
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

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