Avoid using C struct dump for test Ruby code.

Files under `test/t` and `mrbgem/*/test` are for tests, not for actual
execution. So symbols in those files need not to be pre-allocated. This
change slightly reduce the memory consumption.

lib/mruby/build/command.rb

@@ -310,16 +310,16 @@ module MRuby
     def initialize(build)
       super
       @command = nil
-      @compile_options = "-S -B%{funcname} -o-"
+      @compile_options = "-B%{funcname} -o-"
     end
 
-    def run(out, infiles, funcname)
+    def run(out, infiles, funcname, cdump = true)
       @command ||= @build.mrbcfile
       infiles = [infiles].flatten
       infiles.each do |f|
         _pp "MRBC", f.relative_path, nil, :indent => 2
       end
-      cmd = %Q["#{filename @command}" #{@compile_options % {:funcname => funcname}} #{filename(infiles).map{|f| %Q["#{f}"]}.join(' ')}]
+      cmd = %Q["#{filename @command}" #{cdump ? "-S" : ""} #{@compile_options % {:funcname => funcname}} #{filename(infiles).map{|f| %Q["#{f}"]}.join(' ')}]
       puts cmd if Rake.verbose
       IO.popen(cmd, 'r+') do |io|
         out.puts io.read