test: add a test matching what busybox is doing

tests/by-util/test_test.rs

@@ -835,6 +835,15 @@ fn test_or_as_filename() {
     new_ucmd!().args(&["x", "-a", "-z", "-o"]).run().code_is(1);
 }
 
+#[test]
+#[ignore = "TODO: Busybox has this working"]
+fn test_filename_or_with_equal() {
+    new_ucmd!()
+        .args(&["-f", "=", "a", "-o", "b"])
+        .run()
+        .code_is(0);
+}
+
 #[test]
 #[ignore = "GNU considers this an error"]
 fn test_string_length_and_nothing() {