Add spec for the precise report_on_exception output

Author: eregon

spec/ruby/core/thread/report_on_exception_spec.rb

@@ -61,6 +61,36 @@
       }.should raise_error(RuntimeError, "Thread#report_on_exception specs")
     end
 
+    ruby_version_is "3.0" do
+      it "prints a backtrace on $stderr in the regular backtrace order" do
+        line_raise = __LINE__ + 2
+        def foo
+          raise RuntimeError, "Thread#report_on_exception specs backtrace order"
+        end
+
+        line_call_foo = __LINE__ + 5
+        go = false
+        t = Thread.new {
+          Thread.current.report_on_exception = true
+          Thread.pass until go
+          foo
+        }
+
+        -> {
+          go = true
+          Thread.pass while t.alive?
+        }.should output("", <<ERR)
+#{t.inspect} terminated with exception (report_on_exception is true):
+#{__FILE__}:#{line_raise}:in `foo': Thread#report_on_exception specs backtrace order (RuntimeError)
+\tfrom #{__FILE__}:#{line_call_foo}:in `block (5 levels) in <top (required)>'
+ERR
+
+        -> {
+          t.join
+        }.should raise_error(RuntimeError, "Thread#report_on_exception specs backtrace order")
+      end
+    end
+
     it "prints the backtrace even if the thread was killed just after Thread#raise" do
       t = nil
       ready = false