chore: add tests to check against the --test compile flag

Author: sesgoe

tests/ui/exit1_compile_flag_test.rs

@@ -0,0 +1,17 @@
+//@compile-flags: --test
+#![warn(clippy::exit)]
+
+fn not_main() {
+    if true {
+        std::process::exit(4);
+        //~^ exit
+    }
+}
+
+fn main() {
+    if true {
+        std::process::exit(2);
+    };
+    not_main();
+    std::process::exit(1);
+}

tests/ui/exit1_compile_flag_test.stderr

@@ -0,0 +1,11 @@
+error: usage of `process::exit`
+  --> tests/ui/exit1_compile_flag_test.rs:6:9
+   |
+LL |         std::process::exit(4);
+   |         ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::exit` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::exit)]`
+
+error: aborting due to 1 previous error
+

tests/ui/exit2_compile_flag_test.rs

@@ -0,0 +1,15 @@
+//@compile-flags: --test
+#![warn(clippy::exit)]
+
+fn also_not_main() {
+    std::process::exit(3);
+    //~^ exit
+}
+
+fn main() {
+    if true {
+        std::process::exit(2);
+    };
+    also_not_main();
+    std::process::exit(1);
+}

tests/ui/exit2_compile_flag_test.stderr

@@ -0,0 +1,11 @@
+error: usage of `process::exit`
+  --> tests/ui/exit2_compile_flag_test.rs:5:5
+   |
+LL |     std::process::exit(3);
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::exit` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::exit)]`
+
+error: aborting due to 1 previous error
+

tests/ui/exit3_compile_flag_test.rs

@@ -0,0 +1,11 @@
+//@ check-pass
+//@compile-flags: --test
+
+#![warn(clippy::exit)]
+
+fn main() {
+    if true {
+        std::process::exit(2);
+    };
+    std::process::exit(1);
+}