Ignore rbs test failures

For some reason, it's been failing only on YJIT, but apparently it's
reproducible on the interpreter as well. So it's not related to YJIT.

rbs marked rbs tests on Ruby master as allow_failures
ruby/rbs#1536, so it's known to not work on Ruby
master.

We should revert this once we fix the flaky test failure on Ruby master.

.github/workflows/yjit-ubuntu.yml

@@ -162,7 +162,7 @@ jobs:
         timeout-minutes: 60
         env:
           RUBY_TESTOPTS: '-q --tty=no'
-          TEST_BUNDLED_GEMS_ALLOW_FAILURES: ''
+          TEST_BUNDLED_GEMS_ALLOW_FAILURES: 'rbs'
           PRECHECK_BUNDLED_GEMS: 'no'
           SYNTAX_SUGGEST_TIMEOUT: '5'
           YJIT_BINDGEN_DIFF_OPTS: '--exit-code'