cmd/go: don't split internal test main packages twice

Fixes #34321

Change-Id: Ia6253038c525089e20a1da64a2c5c9dcc57edd74
Reviewed-on: https://go-review.googlesource.com/c/go/+/195677
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/load/test.go

@@ -405,7 +405,7 @@ func recompileForTest(pmain, preal, ptest, pxtest *Package) {
 		// covered packages are imported by pmain. Linking multiple packages
 		// compiled with '-p main' causes duplicate symbol errors.
 		// See golang.org/issue/30907, golang.org/issue/34114.
-		if p.Name == "main" && p != pmain {
+		if p.Name == "main" && p != pmain && p != ptest {
 			split()
 		}
 	}

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

+# This test checks that a "main" package with an external test package
+# is recompiled only once.
+# Verifies golang.org/issue/34321.
+
+env GO111MODULE=off
+
+go list -e -test -deps -f '{{if not .Standard}}{{.ImportPath}}{{end}}' pkg
+cmp stdout want
+
+-- $GOPATH/src/pkg/pkg.go --
+package main
+
+func main() {}
+
+-- $GOPATH/src/pkg/pkg_test.go --
+package main
+
+import "testing"
+
+func Test(t *testing.T) {}
+
+-- want --
+pkg
+pkg [pkg.test]
+pkg.test