rustdoc: attempt full build for compile_fail test

Some code fails when doing a full build but does not fail when only
emitting metadata. This commit makes sure compile_fail tests for such
code behave as expected, that is, the test succeeds because the
compilation fails.

Author: tspiteri

src/librustdoc/test.rs

@@ -282,7 +282,7 @@ fn run_test(
     for debugging_option_str in &options.debugging_options_strs {
         compiler.arg("-Z").arg(&debugging_option_str);
     }
-    if no_run {
+    if no_run && !compile_fail {
         compiler.arg("--emit=metadata");
     }
     compiler.arg("--target").arg(target.to_string());