Add basic test

Author: nathanwhit

src/test/ui/lint/lint-temporary-cstring-as-ptr.rs

@@ -0,0 +1,8 @@
+// check-fail
+// ignore-tidy-linelength
+
+use std::ffi::CString;
+
+fn main() {
+    let s = CString::new("some text").unwrap().as_ptr(); //~ ERROR you are getting the inner pointer of a temporary `CString`
+}

src/test/ui/lint/lint-temporary-cstring-as-ptr.stderr

@@ -0,0 +1,16 @@
+error: you are getting the inner pointer of a temporary `CString`
+  --> $DIR/lint-temporary-cstring-as-ptr.rs:7:13
+   |
+LL |     let s = CString::new("some text").unwrap().as_ptr();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(temporary_cstring_as_ptr)]` on by default
+   = note: that pointer will be invalid outside this expression
+help: assign the `CString` to a variable to extend its lifetime
+  --> $DIR/lint-temporary-cstring-as-ptr.rs:7:13
+   |
+LL |     let s = CString::new("some text").unwrap().as_ptr();
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+