cmd/go: convert TestPluginNonMain to script framework
TestPluginNonMain was broken before this change! (It provided the
wrong directory for testdep/p2: testdata/testdep/p2 instead of
testdata/src/testdep/p2).
Change-Id: Ib815f119bae1d758b500cd8ad82c016cb630d71e
Reviewed-on: https://go-review.googlesource.com/c/go/+/212938
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>