Auto merge of #10543 - blyxyas:tests_outside_test_module, r=flip1995

Add `tests_outside_test_module` lint

Adds `tests_outside_test_module` from #10506. This PR **doesn't** close the issue, just resolves task 1.

changelog: [`tests_outside_test_module`]: The lint has been added

tests/ui/tests_outside_test_module.rs

@@ -0,0 +1,18 @@
+// compile-flags: --test
+#![allow(unused)]
+#![warn(clippy::tests_outside_test_module)]
+
+fn main() {
+    // test code goes here
+}
+
+// Should lint
+#[test]
+fn my_test() {}
+
+#[cfg(test)]
+mod tests {
+    // Should not lint
+    #[test]
+    fn my_test() {}
+}

tests/ui/tests_outside_test_module.stderr

@@ -0,0 +1,11 @@
+error: this function marked with #[test] is outside a #[cfg(test)] module
+  --> $DIR/tests_outside_test_module.rs:11:1
+   |
+LL | fn my_test() {}
+   | ^^^^^^^^^^^^^^^
+   |
+   = note: move it to a testing module marked with #[cfg(test)]
+   = note: `-D clippy::tests-outside-test-module` implied by `-D warnings`
+
+error: aborting due to previous error
+