go/internal/gcimporter: in short tests, avoid creating export data for all of std

go/internal/gcimporter: in short tests, avoid creating export data for all of std

gcimporter.TestImportTypeparamTests still needs to create full export
data because it loads lots of source files from GOROOT/test that
expect to be able to import arbitrary subsets of the standard library,
so we now skip it in short mode.

On a clean build cache, this reduces
'go test -short cmd/compile/internal/importer go/internal/gcimporter'
on my machine from 21–28s per test to <6s per test.

Updates #56967.
Updates #47257.

Change-Id: I8fd80293ab135e0d2d213529b74e0ca6429cdfc7
Reviewed-on: https://go-review.googlesource.com/c/go/+/454498
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>