go/internal/gccgoimporter: update importer tests to match std lib version
go/internal/gccgoimporter: update importer tests to match std lib version

Import CL 174738 and CL 164862 from std lib version. This should fix
test failures of issue31540.go on gccgo versions < 7, e.g. on
solaris-amd64-smartosbuildlet

Change-Id: I0a41ca9fe2b248d301fbdf14c9bac3aa3e7b091c
Reviewed-on: https://go-review.googlesource.com/c/tools/+/180897
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

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