cmd/dist: restructure cgo_test
cmd/dist: restructure cgo_test

Currently, dist test has a single test called "cgo_test" that runs a
large number of different "go test"s.

This commit restructures cgo_test into several individual tests, each
of which runs a single "go test" that can be described by a goTest
object and registered with registerTest. Since this lets us raise the
abstraction level of constructing these tests and these tests are
mostly covering the Cartesian product of a small number of orthogonal
dimensions, we pull the common logic for constructing these tests into
a helper function.

For consistency, we now pass -tags=static to the static testtls and
nocgo tests, but this tag doesn't affect the build of these tests at
all. I traced all exec calls from cmd/dist on linux/amd64 and this is
the only non-trivial change.

For #37486.

Change-Id: I53c1efa1c38d785dc71968f05e8d7d636b553e96
Reviewed-on: https://go-review.googlesource.com/c/go/+/450017
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go