Ignore literal_string_with_formatting_args clippy lint in test

    warning: this looks like a formatting argument but it is not part of a formatting macro
       --> tests/test_ensure.rs:505:44
        |
    505 |         "Condition failed: `\"\" == format!(\"{:#?}\", point)`",
        |                                            ^^^^^
        |
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#literal_string_with_formatting_args
        = note: `-W clippy::literal-string-with-formatting-args` implied by `-W clippy::all`
        = help: to override `-W clippy::all` add `#[allow(clippy::literal_string_with_formatting_args)]`

tests/test_ensure.rs

@@ -11,6 +11,7 @@
     clippy::items_after_statements,
     clippy::let_and_return,
     clippy::let_underscore_untyped,
+    clippy::literal_string_with_formatting_args,
     clippy::match_bool,
     clippy::needless_else,
     clippy::never_loop,