cmd/go: fix 'go test foo_test.go'

Fixes race build.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/100410044

src/cmd/go/test.bash

@@ -783,6 +783,12 @@ fi
 rm -f testdata/err.out
 unset GOPATH
 
+TEST 'go test foo_test.go works'
+if ! ./testgo test testdata/standalone_test.go; then
+	echo "go test testdata/standalone_test.go failed"
+	ok=false
+fi
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1

src/cmd/go/test.go

@@ -661,11 +661,14 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 			build: &build.Package{
 				ImportPos: p.build.XTestImportPos,
 			},
-			imports: append(ximports, ptest),
+			imports: ximports,
 			pkgdir:  testDir,
 			fake:    true,
 			Stale:   true,
 		}
+		if ptest != p {
+			pxtest.imports = append(pxtest.imports, ptest)
+		}
 	}
 
 	// Action for building pkg.test.
@@ -675,13 +678,15 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 		GoFiles:    []string{"_testmain.go"},
 		ImportPath: "testmain",
 		Root:       p.Root,
-		imports:    []*Package{ptest},
 		build:      &build.Package{Name: "main"},
 		pkgdir:     testDir,
 		fake:       true,
 		Stale:      true,
 		omitDWARF:  !testC && !testNeedBinary,
 	}
+	if ptest != p {
+		pmain.imports = append(pmain.imports, ptest)
+	}
 	if pxtest != nil {
 		pmain.imports = append(pmain.imports, pxtest)
 	}

src/cmd/go/testdata/standalone_test.go

@@ -0,0 +1,6 @@
+package standalone_test
+
+import "testing"
+
+func Test(t *testing.T) {
+}