internal/imports: skip TestStdlibNotPrefixed
internal/imports: skip TestStdlibNotPrefixed

At tip, $GOROOT/src/go.mod is 1.14, $GOROOT/src/vendor exists, and so
vendor mode is automatically enabled. That causes golang/go#34826.

This will be fixed by my upcoming vendoring CL, but for now skip.

Updates golang/go#34826

Change-Id: I5fff51fff54cf83e6369ae76bf3b19cfb7b5ac15
Reviewed-on: https://go-review.googlesource.com/c/tools/+/203257
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

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