test/ruby/test_optimization.rb: Proc creation test should count :T_DATA

instead of :TOTAL of ObjectSpace.count_objects.

This test had failed very occasionally:

https://rubyci.org/logs/rubyci.s3.amazonaws.com/centos8/ruby-master/log/20200521T033004Z.fail.html.gz
```
  1) Failure:
TestRubyOptimization#test_block_parameter_should_not_create_objects [/home/chkbuild/chkbuild/tmp/build/20200521T033004Z/ruby/test/ruby/test_optimization.rb:713]:
<0> expected but was
<407>.
```

This test of lazy proc creation checks if no object is created during a
method call.  However, calling a method itself increases the count of
objects because method cache is now an object (T_MEMO).
The reason why this test rarely fails is because the test was buggy; it
checked the count of :TOTAL, but :TOTAL count changes only when the GC
heap is expanded.  Creating one object rarely causes heap expansion.
The test must have checked not only :TOTAL but also the count of :FREE.

Instead, this change more directly checks :T_DATA.  Note that a Proc
object is T_DATA.

test/ruby/test_optimization.rb

@@ -710,7 +710,7 @@ class TestRubyOptimization < Test::Unit::TestCase
       foo{}
       ObjectSpace.count_objects(h2)
 
-      assert_equal 0, h2[:TOTAL] - h1[:TOTAL]
+      assert_equal 0, h2[:T_DATA] - h1[:T_DATA] # Proc is T_DATA
     END
   end