Add known-tool-in-submodule test

The test currently fails. The next commit fixes it.

src/test/ui/lint/known-tool-in-submodule/root.rs

@@ -0,0 +1,10 @@
+// check-pass
+
+#![feature(register_tool)]
+#![register_tool(tool)]
+
+mod submodule;
+
+fn main() {
+    submodule::foo();
+}

src/test/ui/lint/known-tool-in-submodule/submodule.rs

@@ -0,0 +1,4 @@
+// ignore-test: not a test
+
+#[allow(tool::lint)]
+pub fn foo() {}