respect current frame of rb_eval_string

`self` is nearest Ruby method's `self`.
If there is no ruby frame, use toplevel `self` (`main`).

https://bugs.ruby-lang.org/issues/18780

test/-ext-/eval/test_eval.rb

@@ -0,0 +1,12 @@
+# frozen_string_literal: false
+require 'test/unit'
+require "-test-/eval"
+
+class  EvalTest < Test::Unit::TestCase
+  def test_rb_eval_string
+    a = 1
+    assert_equal [self, 1, __method__], rb_eval_string(%q{
+      [self, a, __method__]
+    })
+  end
+end