cmd/compile/internal/types2: fix tests on js/wasm

cmd/compile/internal/types2: fix tests on js/wasm

The js/wasm builder started failing as of CL 432535 due to needing
'go build' to import standard-library packages that are no longer
installed to GOROOT/pkg. Since js/wasm can't exec subprocesses,
it cannot run 'go build' to generate the export data needed for
these tests.

For #47257.

Change-Id: I804235fd725faf00b27cbed79ee1f43dea8ab734
Reviewed-on: https://go-review.googlesource.com/c/go/+/446635
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>