x/tools/go/packages/packagestest: TestGOPATHExport and TestModulesExport are failing on Plan 9

[fhs]

--- FAIL: TestGOPATHExport (0.05s)
    gopath_test.go:15: symlink testdata/a.go /tmp/TestGOPATHExport3852585505/fake1/src/golang.org/fake1/a.go: not supported by plan 9
--- FAIL: TestModulesExport (0.01s)
    modules_test.go:17: symlink testdata/a.go /tmp/TestModulesExport828457641/fake1/a.go: not supported by plan 9
FAIL
FAIL	golang.org/x/tools/go/packages/packagestest	5.750s

(from https://storage.googleapis.com/go-build-log/27255227/plan9-arm_854b9959.log)

@gopherbot add labels OS-Plan9, NeedsInvestigation

[gopherbot]

Change https://golang.org/cl/234112 mentions this issue: go/packages/packagestest: make Export skip tests involving unsupported links

[bcmills]

I already had a fix for this ready in CL 234112, so I just dusted it off and I'll submit it if it passes TryBots.