translate: keep _test.go suffix when mangling

It has meaning that should be maintained.

internal/translate/testdata/constraints.translate.txt

@@ -41,13 +41,13 @@ package basicgosim
 
 func Hello() {
 }
--- hello_linux_test.go --
+-- translated/constraints/hello_linux__test.go --
+// Code generated by gosim. DO NOT EDIT.
 package basicgosim_test
 
 func Hello() {
 }
--- translated/constraints/hello_linux_test_.go --
-// Code generated by gosim. DO NOT EDIT.
+-- hello_linux_test.go --
 package basicgosim_test
 
 func Hello() {

internal/translate/translate.go

@@ -112,6 +112,9 @@ func renameFile(cfg gosimtool.BuildConfig, filePath string) string {
 
 	// for now, after translate all files should be included in the build
 	// modify the filename minimally to neuter any excluded files
+	if strings.HasSuffix(withoutDot, "_test") {
+		return strings.TrimSuffix(withoutDot, "_test") + "__test" + ext
+	}
 	return withoutDot + "_" + ext
 }
 

internal/translate/translate_test.go

@@ -243,15 +243,15 @@ func TestRenameFile(t *testing.T) {
 		},
 		{
 			in:  "foo/bar_linux_test.go",
-			out: "foo/bar_linux_test_.go",
+			out: "foo/bar_linux__test.go",
 		},
 		{
 			in:  "foo/bar_linux_amd64.go",
 			out: "foo/bar_linux_amd64_.go",
 		},
 		{
 			in:  "foo/bar_linux_amd64_test.go",
-			out: "foo/bar_linux_amd64_test_.go",
+			out: "foo/bar_linux_amd64__test.go",
 		},
 		{
 			in:  "foo/bar_amd64.go",