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>

src/cmd/go/go_test.go

@@ -2281,20 +2281,6 @@ func TestCoverageDashC(t *testing.T) {
 	tg.wantExecutable(tg.path("coverdep"), "go -test -c -coverprofile did not create executable")
 }
 
-func TestPluginNonMain(t *testing.T) {
-	wd, err := os.Getwd()
-	if err != nil {
-		t.Fatal(err)
-	}
-
-	pkg := filepath.Join(wd, "testdata", "testdep", "p2")
-
-	tg := testgo(t)
-	defer tg.cleanup()
-
-	tg.runFail("build", "-buildmode=plugin", pkg)
-}
-
 func TestTestEmpty(t *testing.T) {
 	if !canRace {
 		t.Skip("no race detector")

src/cmd/go/testdata/script/build_plugin_non_main.txt

@@ -0,0 +1,17 @@
+# Plugins are only supported on linux,cgo and darwin,cgo.
+[!linux] [!darwin] skip
+[!cgo] skip
+
+go build -n testdep/p2
+! go build -buildmode=plugin testdep/p2
+stderr '-buildmode=plugin requires exactly one main package'
+
+-- testdep/p1/p1.go --
+package p1
+-- testdep/p1/p1_test.go --
+package p1
+
+import _ "testdep/p2"
+-- testdep/p2/p2.go --
+package p2
+