don't break TestMain funcs

Important for 'garble test', if a package uses one.

main.go

@@ -363,7 +363,7 @@ func transformGo(node ast.Node, info *types.Info) ast.Node {
 					return true // might implement an interface
 				}
 				switch node.Name {
-				case "main", "init":
+				case "main", "init", "TestMain":
 					return true // don't break them
 				}
 				if strings.HasPrefix(node.Name, "Test") && isTestSignature(sign) {

testdata/scripts/test.txt

@@ -45,3 +45,14 @@ func TestSeparateFoo(t *testing.T) {
 		t.FailNow()
 	}
 }
+-- main_test.go --
+package bar
+
+import (
+	"os"
+	"testing"
+)
+
+func TestMain(m *testing.M) {
+	os.Exit(m.Run())
+}