util/testenv: add ArtifactDir, Attr, Output methods to TB

The TB interface exists to mirror testing.TB without importing the
testing package, but it had fallen behind: Go 1.25 added Attr and
Output, and Go 1.26 added ArtifactDir. Add the missing methods and a
reflection-based test that TB has every exported method of testing.TB,
so future additions to testing.TB fail a test instead of silently
diverging. It can't be a compile-time assertion because testing.TB has
an unexported method.

Updates #16330
Updates #18682

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I9ba093afefdf3a6311ef4648bc1a13add9af453d