cmd/go/internal/test: refresh flagdefs.go and fix test

cmd/go/internal/test: refresh flagdefs.go and fix test

The tests for cmd/go/internal/test were not running at all due to a
missed call to m.Run in TestMain. That masked two missing vet
analyzers ("directive" and "timeformat") and a missed update to the
generator script in CL 355452.

Fixes #58415.

Change-Id: I7b0315952967ca07a866cdaa5903478b2873eb7a
Reviewed-on: https://go-review.googlesource.com/c/go/+/466635
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>