internal/imports: skip TestModeGetmodeVendor

Test is broken by recent cmd/go vendoring changes. Skipping the test
case while we figure out what to do.

Updates golang/go#34826

Change-Id: I31a6f18e40420970251e25836edcf32c56c10ef5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/200299
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

internal/imports/mod_test.go

@@ -216,6 +216,7 @@ import _ "rsc.io/quote"
 
 // Tests that -mod=vendor sort of works. Adapted from mod_getmode_vendor.txt.
 func TestModeGetmodeVendor(t *testing.T) {
+	t.Skip("'go list -m -mod=vendor' currently not allowed: see golang.org/issue/34826")
 	mt := setup(t, `
 -- go.mod --
 module x