rustdoc use -Ccodegen-units=1 by default for test compile

as the test is small we do not want split up in multiple codegen units
and also as there is multiple test running at the same time this
will reduce the number of concurrent threads

src/librustdoc/test.rs

@@ -263,6 +263,7 @@ fn run_test(
     for extern_str in &options.extern_strs {
         compiler.arg("--extern").arg(&extern_str);
     }
+    compiler.arg("-Ccodegen-units=1");
     for codegen_options_str in &options.codegen_options_strs {
         compiler.arg("-C").arg(&codegen_options_str);
     }