Restore TestNonEmptyMountPoint for Linux.

I had forgotten that we explicitly handle the "mount point doesn't
exist" case, so that wasn't tickling what I had hoped in fusermount.

mount_test.go

@@ -4,6 +4,7 @@ import (
 	"io/ioutil"
 	"os"
 	"path"
+	"runtime"
 	"strings"
 	"testing"
 
@@ -65,6 +66,51 @@ func TestSuccessfulMount(t *testing.T) {
 	defer fuse.Unmount(mfs.Dir())
 }
 
+func TestNonEmptyMountPoint(t *testing.T) {
+	ctx := context.Background()
+
+	// osxfuse appears to be happy to mount over a non-empty mount point.
+	//
+	// We leave this test in for Linux, because it tickles the behavior of
+	// fusermount writing to stderr and exiting with an error code. We want to
+	// make sure that a descriptive error makes it back to the user.
+	if runtime.GOOS == "darwin" {
+		return
+	}
+
+	// Set up a temporary directory.
+	dir, err := ioutil.TempDir("", "mount_test")
+	if err != nil {
+		t.Fatal("ioutil.TempDir: %v", err)
+	}
+
+	defer os.RemoveAll(dir)
+
+	// Add a file within it.
+	err = ioutil.WriteFile(path.Join(dir, "foo"), []byte{}, 0600)
+	if err != nil {
+		t.Fatalf("ioutil.WriteFile: %v", err)
+	}
+
+	// Attempt to mount.
+	fs := &minimalFS{}
+	mfs, err := fuse.Mount(
+		dir,
+		fuseutil.NewFileSystemServer(fs),
+		&fuse.MountConfig{})
+
+	if err == nil {
+		fuse.Unmount(mfs.Dir())
+		mfs.Join(ctx)
+		t.Fatal("fuse.Mount returned nil")
+	}
+
+	const want = "not empty"
+	if got := err.Error(); !strings.Contains(got, want) {
+		t.Errorf("Unexpected error: %v", got)
+	}
+}
+
 func TestNonexistentMountPoint(t *testing.T) {
 	ctx := context.Background()