misc/cgo/testso{,var}: fix tests in module mode

Add _test.go files in the individal directories to invoke 'go build'
with appropriate arguments.

Move the test driver out of cmd/dist so that it's easier to invoke the
test separately (using 'go test .').

Updates #30228
Updates #28387

Change-Id: Ibc4a024a52c12a274058298b41cc90709f7f56c8
Reviewed-on: https://go-review.googlesource.com/c/163420


Reviewed-by: Ian Lance Taylor <iant@golang.org>