Rollup merge of rust-lang#52627 - Mark-Simulacrum:rustdoc-test-correct, r=alexcrichton

Compile rustc before building tests for rustdoc

r? @alexcrichton

Author: kennytm

src/bootstrap/test.rs

@@ -1732,6 +1732,7 @@ impl Step for CrateRustdoc {
 
         let compiler = builder.compiler(builder.top_stage, self.host);
         let target = compiler.host;
+        builder.ensure(compile::Rustc { compiler, target });
 
         let mut cargo = tool::prepare_tool_cargo(builder,
                                                  compiler,