Make rustdoc output deterministic for UI tests

Author: jyn514

src/test/rustdoc-ui/failed-doctest-output.rs

@@ -2,7 +2,7 @@
 // FIXME: if/when the output of the test harness can be tested on its own, this test should be
 // adapted to use that, and that normalize line can go away
 
-// compile-flags:--test
+// compile-flags:--test --test-args --test-threads=1
 // rustc-env:RUST_BACKTRACE=0
 // normalize-stdout-test: "src/test/rustdoc-ui" -> "$$DIR"
 // failure-status: 101