Add separate program for locking state files

Depending on the implementation, local state locks may be reentrant
within the same process. Use a separate process to test locked state
files.

command/testdata/statelocker.go

@@ -0,0 +1,43 @@
+// statelocker use used for testing command with a locked state.
+// This will lock the state file at a given path, then wait for a sigal. On
+// SIGINT and SIGTERM the state will be Unlocked before exit.
+package main
+
+import (
+	"io"
+	"log"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/hashicorp/terraform/state"
+)
+
+func main() {
+	if len(os.Args) != 2 {
+		log.Fatal(os.Args[0], "statefile")
+	}
+
+	s := &state.LocalState{
+		Path: os.Args[1],
+	}
+
+	err := s.Lock("command test")
+	if err != nil {
+		io.WriteString(os.Stderr, err.Error())
+		return
+	}
+
+	// signal to the caller that we're locked
+	io.WriteString(os.Stdout, "LOCKED")
+
+	defer func() {
+		if err := s.Unlock(); err != nil {
+			io.WriteString(os.Stderr, err.Error())
+		}
+	}()
+
+	c := make(chan os.Signal, 1)
+	signal.Notify(c, syscall.SIGINT, syscall.SIGTERM)
+	<-c
+}