cmd/gofmt: skip gofmt idempotency check on known issue

gofmt's TestAll runs gofmt on all the go files in the tree and checks,
among other things, that gofmt is idempotent (i.e. that a second
invocation does not change the input again).

There's a known bug of gofmt not being idempotent (Issue #24472), and
unfortunately the fixedbugs/issue22662.go file triggers it. We can't
just gofmt the file, because it tests the effect of various line
directives inside weirdly-placed comments, and gofmt moves those
comments, making the test useless.

Instead, just skip the idempotency check when gofmt-ing the
problematic file.

This fixes go test on the cmd/gofmt package, and a failure seen on the
longtest builder.

Updates #24472

Change-Id: Ib06300977cd8fce6c609e688b222e9b2186f5aa7
Reviewed-on: https://go-review.googlesource.com/130377


Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>