Add ui test for include_file_outside_project lint

Author: GuillaumeGomez

tests/ui/include_file_outside_project.rs

@@ -0,0 +1,9 @@
+#![deny(clippy::include_file_outside_project)]
+
+// Should not lint.
+include!("./auxiliary/external_consts.rs");
+
+fn main() {
+    let x = include_str!(concat!(env!("CARGO_HOME"), "/.crates.toml"));
+    //~^ include_file_outside_project
+}

tests/ui/include_file_outside_project.stderr

@@ -0,0 +1,15 @@
+error: attempted to include a file outside of the project
+  --> tests/ui/include_file_outside_project.rs:7:13
+   |
+LL |     let x = include_str!(concat!(env!("CARGO_HOME"), "/.crates.toml"));
+   |             ^
+   |
+   = note: file is located at `/home/imperio/.cargo/.crates.toml` which is outside of project folder (`/home/imperio/rust/clippy`)
+note: the lint level is defined here
+  --> tests/ui/include_file_outside_project.rs:1:9
+   |
+LL | #![deny(clippy::include_file_outside_project)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+