Run mrbgem and core tests on minimum dependencies.

Solves #2355.

In test drivers:
* Uses `mrb_t_pass_result` to check and pass test result to main `mrb_state`.
* Adds `mrb_init_test_driver` to init test `mrb_state`.

test/mrbtest.rake

@@ -12,11 +12,12 @@ MRuby.each_target do
   ass_lib = ass_c.ext(exts.object)
 
   mrbtest_lib = libfile("#{current_build_dir}/mrbtest")
-  gem_test_files = gems.select { |g| g.run_test_in_other_mrb_state? }.map { |g| g.test_rbireps.ext(exts.object) }
-  file mrbtest_lib => [mlib, ass_lib, gems.map(&:test_objs), gem_test_files].flatten do |t|
+  mrbtest_objs = gems.map do |v|
+    v.test_objs + [v.test_rbireps.ext(exts.object)]
+  end
+  file mrbtest_lib => [mlib, ass_lib] + mrbtest_objs do |t|
     archiver.run t.name, t.prerequisites
   end
-  file mrbtest_lib => "#{build_dir}/test/no_mrb_open_test.c".ext(exts.object)
 
   unless build_mrbtest_lib_only?
     driver_obj = objfile("#{current_build_dir}/driver")
@@ -55,16 +56,12 @@ MRuby.each_target do
       f.puts IO.read(init)
       mrbc.run f, mrbs, 'mrbtest_irep'
       gems.each do |g|
-        next unless g.run_test_in_other_mrb_state?
         f.puts %Q[void GENERATED_TMP_mrb_#{g.funcname}_gem_test(mrb_state *mrb);]
       end
-      f.puts %Q[void no_mrb_open_mrbgem_test(mrb_state *mrb);]
       f.puts %Q[void mrbgemtest_init(mrb_state* mrb) {]
       gems.each do |g|
-        next unless g.run_test_in_other_mrb_state?
         f.puts %Q[    GENERATED_TMP_mrb_#{g.funcname}_gem_test(mrb);]
       end
-      f.puts %Q[    no_mrb_open_mrbgem_test(mrb);]
       f.puts %Q[}]
     end
   end