Support `attr` action

[DaltonSW]

Added in Go 1.25.0

From the release notes:

The new methods T.Attr, B.Attr, and F.Attr emit an attribute to the test log. An attribute is an arbitrary key and value associated with a test.

For example, in a test named TestF, t.Attr("key", "value") emits:

=== ATTR TestF key value

With the -json flag, attributes appear as a new “attr” action.