Use tests instead of $DIR in stderr expected output

Author: ithinuel

tests/ui/unused_result_ok.stderr

@@ -1,5 +1,5 @@
 error: ignoring a result with `.ok()` is misleading
-  --> $DIR/unused_result_ok.rs:9:5
+  --> tests/ui/unused_result_ok.rs:9:5
    |
 LL |     x.parse::<u32>().ok();
    |     ^^^^^^^^^^^^^^^^^^^^^
@@ -12,7 +12,7 @@ LL |     let _ = x.parse::<u32>();
    |     ~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: ignoring a result with `.ok()` is misleading
-  --> $DIR/unused_result_ok.rs:18:5
+  --> tests/ui/unused_result_ok.rs:18:5
    |
 LL |     x   .   parse::<i32>()   .   ok   ();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -23,7 +23,7 @@ LL |     let _ = x   .   parse::<i32>();
    |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 error: ignoring a result with `.ok()` is misleading
-  --> $DIR/unused_result_ok.rs:34:5
+  --> tests/ui/unused_result_ok.rs:34:5
    |
 LL |     v!().ok();
    |     ^^^^^^^^^
@@ -34,7 +34,7 @@ LL |     let _ = v!();
    |     ~~~~~~~~~~~~
 
 error: ignoring a result with `.ok()` is misleading
-  --> $DIR/unused_result_ok.rs:29:9
+  --> tests/ui/unused_result_ok.rs:29:9
    |
 LL |         Ok::<(), ()>(()).ok();
    |         ^^^^^^^^^^^^^^^^^^^^^