Use ui test syntax that does not interfere with rustfmt

test_suite/tests/ui/conflict/alias-enum.stderr

@@ -1,8 +1,8 @@
-error: expected expression, found `@`
+error[E0425]: cannot find value `__FAIL__` in this scope
   --> tests/ui/conflict/alias-enum.rs:74:5
    |
-74 |     @//fail
-   |     ^ expected expression
+74 |     __FAIL__;
+   |     ^^^^^^^^ not found in this scope
 
 warning: unreachable pattern
   --> tests/ui/conflict/alias-enum.rs:13:9