misc/cgo/test: consolidate tests into fewer cgo source files

misc/cgo/test: consolidate tests into fewer cgo source files

Each different file that does import "C" must be compiled
and analyzed separately by cgo. Having fewer files import "C"
reduces the cost of building the test. This is especially important
because this test is built and run four different times (with different
settings) during all.bash.

go test -c in this directory used to take over 20 seconds on my laptop.
Now it takes under 5 seconds.

Removes 23.4r 29.0u 21.5s from all.bash.

For #26473.

Change-Id: Ie7cb7b0d9d6138ebd2eb548d0d8ea6e409ae10b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/177558
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>