Build a single binary containing all the TestPointerChecks
instead of building many small binaries,
each with its own cgo+compile+link invocation.
This cuts 'go test -run=TestPointerChecks'
from 6.7r 35.5u 26.1s to 2.1r 2.1u 1.4s.

Move as many cgo checks as possible into fewer test files
for TestReportsTypeErrors too.
This cuts 'go test -run=TestReportsTypeErrors'
from 2.1r 6.7u 6.7s to 1.5r 2.5u 2.5s.

After this change, all.bash runs in ~4:30 on my laptop.

For #26473.

Change-Id: I3787448b03689a1f62dd810957ab6013bb75582f
Reviewed-on: https://go-review.googlesource.com/c/go/+/177599
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>