[GR-32856] Fix --cpusampler test

* #itself does not execute the block, #yield_self and #then do.

Author: eregon

spec/truffle/tools_spec.rb

@@ -59,7 +59,7 @@
       code = <<~RUBY
       def foo
         n = 0
-        loop { itself { n += 1 } }
+        loop { yield_self { n += 1 } }
         n
       end
       t = Thread.new { foo }
@@ -70,7 +70,7 @@ def foo
       RUBY
       out = ruby_exe(code, options: "--cpusampler")
       out.should.include?(":kill")
-      out.should.include?("block in Object#foo")
+      out.should.include?("block (2 levels) in Object#foo")
       out.should_not.include?('KillException')
       $?.should.success?
     end