Add tests for mruby-eval.

mrbgems/mruby-eval/test/eval.rb

@@ -0,0 +1,8 @@
+assert('Kernel.eval') do
+  assert_equal(10) { Kernel.eval '1 * 10' }
+  assert_equal('aaa') { Kernel.eval "'a' * 3" }
+end
+
+assert('eval') do
+  assert_equal(10) { eval '1 * 10' }
+end