misc/cgo/testsigfwd: move to runtime/testprog/testprogcgo
This migrates testsigfwd, which uses some one-off build
infrastructure, to be part of the runtime's testprogcgo.
The test is largely unchanged. Because it's part of a larger binary,
this CL renames a few things and gates the constructor-time signal
handler registration on an environment variable. This CL also replaces
an errant fmt.Errorf with fmt.Fprintf.
For #37486, since it eliminates a non-go-test from dist.
Change-Id: I0efd146ea0a0a3f0b361431349a419af0f0ecc61
Reviewed-on: https://go-review.googlesource.com/c/go/+/443068
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>