Fix test case

Previously this was using a `.stderr` file, but that does not seem to
work for all cases. This change uses `// error-pattern:` instead.

Author: eholk

src/test/ui/lint/command-line-register-unknown-lint-tool.rs

@@ -1,4 +1,4 @@
 // compile-flags: -A unknown_tool::foo
-// check-fail
+// error-pattern: unknown lint tool: `unknown_tool`
 
 fn main() {}