tests: improve test coverage

Improve test coverage as per review feedback.

wait/walk_test.go

@@ -57,6 +57,7 @@ func TestWalk(t *testing.T) {
 		require.Equal(t, 2, matched)
 
 		count = 0
+		matched = 0
 		err = wait.Walk(&req.WaitingFor, func(s wait.Strategy) error {
 			count++
 			if _, ok := s.(*wait.FileStrategy); ok {
@@ -66,6 +67,7 @@ func TestWalk(t *testing.T) {
 		})
 		require.NoError(t, err)
 		require.Equal(t, 3, count)
+		require.Zero(t, matched)
 	})
 
 	t.Run("remove-stop", func(t *testing.T) {
@@ -84,4 +86,40 @@ func TestWalk(t *testing.T) {
 		require.Equal(t, 1, count)
 		require.Nil(t, req.WaitingFor)
 	})
+
+	t.Run("nil-root", func(t *testing.T) {
+		err := wait.Walk(nil, func(_ wait.Strategy) error {
+			return nil
+		})
+		require.EqualError(t, err, "root strategy is nil")
+	})
+
+	t.Run("direct-single", func(t *testing.T) {
+		req := testcontainers.ContainerRequest{
+			WaitingFor: wait.ForFile("/tmp/file"),
+		}
+		requireVisits(t, req, 1)
+	})
+
+	t.Run("for-all-single", func(t *testing.T) {
+		req := testcontainers.ContainerRequest{
+			WaitingFor: wait.ForAll(
+				wait.ForFile("/tmp/file"),
+			),
+		}
+		requireVisits(t, req, 2)
+	})
+}
+
+// requireVisits validates the number of visits for a given request.
+func requireVisits(t *testing.T, req testcontainers.ContainerRequest, expected int) {
+	t.Helper()
+
+	var count int
+	err := wait.Walk(&req.WaitingFor, func(_ wait.Strategy) error {
+		count++
+		return nil
+	})
+	require.NoError(t, err)
+	require.Equal(t, expected, count)
 }