go/internal/gcimporter: add test of export/import on std
go/internal/gcimporter: add test of export/import on std

This test ensures that all std and x/tools packages can
be re-typechecked using export data.

Change-Id: I189a8138d74cb38f69dfb51c613849e43b316322
Reviewed-on: https://go-review.googlesource.com/c/tools/+/445096
Reviewed-by: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools