Add ui tests for UNNECESSARY_TO_OWNED on split

tests/ui/unnecessary_to_owned_on_split.fixed

@@ -0,0 +1,6 @@
+fn main() {
+    let _ = "a".split('a').next().unwrap();
+    //~^ ERROR: unnecessary use of `to_string`
+    let _ = "a".split('a').next().unwrap();
+    //~^ ERROR: unnecessary use of `to_owned`
+}

tests/ui/unnecessary_to_owned_on_split.rs

@@ -0,0 +1,6 @@
+fn main() {
+    let _ = "a".to_string().split('a').next().unwrap();
+    //~^ ERROR: unnecessary use of `to_string`
+    let _ = "a".to_owned().split('a').next().unwrap();
+    //~^ ERROR: unnecessary use of `to_owned`
+}

tests/ui/unnecessary_to_owned_on_split.stderr

@@ -0,0 +1,17 @@
+error: unnecessary use of `to_string`
+  --> $DIR/unnecessary_to_owned_on_split.rs:2:13
+   |
+LL |     let _ = "a".to_string().split('a').next().unwrap();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `"a".split('a')`
+   |
+   = note: `-D clippy::unnecessary-to-owned` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::unnecessary_to_owned)]`
+
+error: unnecessary use of `to_owned`
+  --> $DIR/unnecessary_to_owned_on_split.rs:4:13
+   |
+LL |     let _ = "a".to_owned().split('a').next().unwrap();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ help: use: `"a".split('a')`
+
+error: aborting due to 2 previous errors
+