cmd/cgo/internal/test: add a test for C calling into Go before init done

Test that C code can call back into Go (on a separate thread)
before init is done. The callback should wait for the init done.
And this should not cause a false race.

This corresponds to the runtime fix in CL 746581.

Change-Id: I10e6a9d5fe056be16143f5f6a322f51e56fc9fdf
Reviewed-on: https://go-review.googlesource.com/c/go/+/746780
Reviewed-by: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>