Exclude debuginfo tests from bors

Stepping through a debugger is more difficult now Boehm has its own
SIGSEGV handler. These tests are deprecated anyway, and the rust team
are working on removing them.

More info: rust-lang#47163

Author: jacob-hughes

.buildbot.sh

@@ -7,4 +7,4 @@ set -e
 # Ensure the build fails if it uses excessive amounts of memory.
 ulimit -d $((1024 * 1024 * 8)) # 8 GiB
 
-./x.py test src/libstd --stage 1
+./x.py test --stage 1 --exclude rustdoc-json --exclude debuginfo