Prevent warning: assigned but unused variable

http://rubyci.s3.amazonaws.com/debian10/ruby-master/log/20230510T123003Z.log.html.gz
```
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/objspace/test_objspace.rb:224: warning: assigned but unused variable - c4
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/ruby/test_class.rb:362: warning: assigned but unused variable - e
/home/chkbuild/chkbuild/tmp/build/20230510T123003Z/ruby/test/ruby/test_process.rb:2602: warning: assigned but unused variable - parent_pid
```

test/objspace/test_objspace.rb

@@ -221,7 +221,7 @@ class TestObjSpace < Test::Unit::TestCase
       o4 =
         # This line intentionally left blank
         # This line intentionally left blank
-        1.0 / 0.0; line4 = __LINE__; c4 = GC.count
+        1.0 / 0.0; line4 = __LINE__; _c4 = GC.count
       assert_equal(__FILE__, ObjectSpace.allocation_sourcefile(o4))
       assert_equal(line4, ObjectSpace.allocation_sourceline(o4))
 

test/ruby/test_class.rb

@@ -359,7 +359,7 @@ class TestClass < Test::Unit::TestCase
     assert_raise_with_message NameError, /uninitialized/ do
       begin
         eval('class ::TestClass::PrivateClass; end')
-      rescue NameError => e
+      rescue NameError
       end
 
       Object.const_get "NOT_AVAILABLE_CONST_NAME_#{__LINE__}"

test/ruby/test_process.rb

@@ -2599,7 +2599,7 @@ EOS
   end if Process.respond_to?(:_fork)
 
   def test__fork_pid_cache
-    parent_pid = Process.pid
+    _parent_pid = Process.pid
     r, w = IO.pipe
     pid = Process._fork
     if pid == 0