Previously, the behavior of ShouldRunDistTest could be overridden
by an individual builder by setting a the shouldRunDistTest field
to a custom policy implementation.

A pitfall of this approach is that it requires each custom policy
to reimplement the default cmd/dist test policy as well. If that
isn't done when attempting to make a small adjustment to the default
policy, the result is that the default policy is ignored.

The fasterTrybots policy, as the name suggests, exists to skip some
tests in trybot mode. Due to the problem above, it was causing some
builders to run more tests in trybot mode than they would if it
weren't applied. For example, the freebsd-amd64-12_0 builder should
have been skipping the "api" test in trybot mode, but wasn't.

This refactor fixes that by modifying the approach for how individual
builders can customize the cmd/dist test behavior. Now, a function
distTestAdjust is called with the first parameter specifying the
default policy decision. That function can adjust the decision as it
sees fit, but otherwise it preserves its initial value in situations
where no change is intended to be made.

For golang/go#38279.

Change-Id: I220248f51abfc8e77195ae0c9134a17f7cac9bc2
Reviewed-on: https://go-review.googlesource.com/c/build/+/227777
Reviewed-by: Alexander Rakoczy <alex@golang.org>

Previously, a builder was considered to be "SlowBot" whenever it was
requested via the TRY= syntax, and it wasn't already a part of the
default set of trybot builders.

This change makes it so that a builder is considered a SlowBot if
it was explicitly requested via the TRY= syntax, even if it was
already going to run anyway.

The goal behind this change is to improve the experience of using
SlowBots, by making them more useful and predictable.

The UI currently reports that "SlowBots are starting ..." whenever
there is a non-zero amount of SlowBots. So if a user requested
TRY=linux-amd64, they'd likely see "TryBots are starting ...",
which can be misunderstood to mean that SlowBots are not working.

Additionally, if a user requested three builders via TRY= syntax,
but one of them happened to be a part of the default try set, then
the message at the end would be "Extra slowbot builds that ran:"
including only 2 builders. At that point, it's hard to tell whether
they got the TRY= syntax wrong for one of the builders, or if it was
omitted because it's always run anyway.

A future usability improvement in golang.org/issue/38279 is to make
SlowBots not skip any tests that normal trybots may skip for faster
results. This change is in preparation for that.

For golang/go#38279.
For golang/go#34501.

Change-Id: Idb7f1bcb1694fe72f85237976995a94f273c7c16
Reviewed-on: https://go-review.googlesource.com/c/build/+/227778
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>