rustdoc: Test only in stage2

stage1's rustdoc is using stage0's libtest which does not have
a fix from 069cf9dfc9, making the test run fail. Ensure that
this test is executed with everything recompiled in stage2.

tests/run-make/doctests-junit/rmake.rs

@@ -1,6 +1,7 @@
 // Check rustdoc's test JUnit (XML) output against snapshots.
 
 //@ ignore-cross-compile (running doctests)
+//@ ignore-stage1 (rustdoc depends on a fix in libtest)
 //@ needs-unwind (test file contains `should_panic` test)
 
 use std::path::Path;