Tag failing thread interrupt spec

The way we have implemented thread interrupts, it's possible for
a second event to overtake the propagation of a first event,
leading to failures in this spec:

Thread#report_on_exception= when set to true prints the backtrace even if the thread was killed just after Thread#raise

This tags that spec for now and we will explore how to fix it in
jruby#8479.

spec/tags/ruby/core/thread/report_on_exception_tags.txt

@@ -1 +1,2 @@
 fails:Thread#report_on_exception= when set to true prints a backtrace on $stderr in the regular backtrace order
+fails:Thread#report_on_exception= when set to true prints the backtrace even if the thread was killed just after Thread#raise