test/ruby/test_rubyvm.rb: prevent "assigned but unused variable" warnings

http://rubyci.s3.amazonaws.com/centos7/ruby-master/log/20211025T093004Z.log.html.gz
```
[ 4896/21159] TestRubyVM#test_keep_script_lines(none):3: warning: assigned but unused variable - b
(none):6: warning: assigned but unused variable - c
(none):1: warning: assigned but unused variable - a
<compiled>:3: warning: assigned but unused variable - b
(none):3: warning: assigned but unused variable - b
(none):6: warning: assigned but unused variable - c
(none):1: warning: assigned but unused variable - a
<compiled>:3: warning: assigned but unused variable - b
```

test/ruby/test_rubyvm.rb

@@ -18,12 +18,12 @@ class TestRubyVM < Test::Unit::TestCase
 
   def parse_and_compile
     script = <<~RUBY
-      a = 1
+      _a = 1
       def foo
-        b = 2
+        _b = 2
       end
       1.times{
-        c = 3
+        _c = 3
       }
     RUBY