mruby-complex: add test for division by zero

Add a test case to ensure that dividing a complex number by
`Complex(0, 0)` raises a `ZeroDivisionError`.

Co-authored-by: Gemini <gemini@google.com>

mrbgems/mruby-complex/test/complex.rb

@@ -76,6 +76,8 @@ assert 'Complex#/' do
     one = 1e200
   end
   assert_complex Complex(ten, ten) / Complex(one, one), Complex(10.0, 0.0)
+  assert_raise(ZeroDivisionError) { Complex(1,1) / 0 }
+  assert_raise(ZeroDivisionError) { Complex(1,1) / Complex(0,0) }
 end
 
 assert 'Complex#==' do