Add test for option_map_or_err_ok lint

tests/ui/option_map_or_err_ok.fixed

@@ -0,0 +1,7 @@
+#![warn(clippy::option_map_or_err_ok)]
+
+fn main() {
+    let x = Some("a");
+    let _: Result<&str, &str> = x.ok_or("a");
+    //~^ ERROR: called `map_or(Err(_), Ok)` on an `Option` value
+}

tests/ui/option_map_or_err_ok.rs

@@ -0,0 +1,7 @@
+#![warn(clippy::option_map_or_err_ok)]
+
+fn main() {
+    let x = Some("a");
+    let _: Result<&str, &str> = x.map_or(Err("a"), Ok);
+    //~^ ERROR: called `map_or(Err(_), Ok)` on an `Option` value
+}

tests/ui/option_map_or_err_ok.stderr

@@ -0,0 +1,11 @@
+error: called `map_or(Err(_), Ok)` on an `Option` value
+  --> $DIR/option_map_or_err_ok.rs:5:33
+   |
+LL |     let _: Result<&str, &str> = x.map_or(Err("a"), Ok);
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^ help: try using `ok_or` instead: `x.ok_or("a")`
+   |
+   = note: `-D clippy::option-map-or-err-ok` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::option_map_or_err_ok)]`
+
+error: aborting due to previous error
+