cmd/go: test: do not put object files where later steps will find them

When recompiling a package whose basename is the name of a standard
package for testing with gccgo, a .o file with the basename of the
package being tested was being placed in the _test/ directory where the
compilation of the test binary then found it when looking for the
standard library package.

This change puts the object files in a separate directory.

Fixes #6793

R=golang-codereviews, dave, gobot, rsc, iant
CC=golang-codereviews
https://golang.org/cl/27650045

src/cmd/go/test.go

@@ -711,7 +711,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 
 	if ptest != p {
 		a := b.action(modeBuild, modeBuild, ptest)
-		a.objdir = testDir + string(filepath.Separator)
+		a.objdir = testDir + string(filepath.Separator) + "_obj_test" + string(filepath.Separator)
 		a.objpkg = ptestObj
 		a.target = ptestObj
 		a.link = false
@@ -719,7 +719,7 @@ func (b *builder) test(p *Package) (buildAction, runAction, printAction *action,
 
 	if pxtest != nil {
 		a := b.action(modeBuild, modeBuild, pxtest)
-		a.objdir = testDir + string(filepath.Separator)
+		a.objdir = testDir + string(filepath.Separator) + "_obj_xtest" + string(filepath.Separator)
 		a.objpkg = buildToolchain.pkgpath(testDir, pxtest)
 		a.target = a.objpkg
 	}