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.

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

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

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

-error[E0602]: unknown lint tool: `unknown_tool`
-   |
-   = note: requested on the command line with `-A foo`
-
-error[E0602]: unknown lint tool: `unknown_tool`
-   |
-   = note: requested on the command line with `-A foo`
-
-error: aborting due to 2 previous errors
-
-For more information about this error, try `rustc --explain E0602`.