go/internal/gcimporter: skip TestIExportData_stdlib on go1.18
go/internal/gcimporter: skip TestIExportData_stdlib on go1.18

This test will fail on Go 1.18 once standard library changes are
introduced that use generics. Skip it until iexport.go has been updated
to support generics.

Updates golang/go#48392

Change-Id: I0f779689daaf2bc3127affff633c883ed9d7289b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/349954
Trust: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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