Add tests for String Ranges

Range#each depends on String#upto which is implemented in mruby-string-ext
which is why these tests live there.

mrbgems/mruby-string-ext/test/range.rb

+assert('Range#max') do
+  # string Range depends on String#upto
+  assert_equal 'l', ('f'..'l').max
+  assert_equal 'e', ('a'...'f').max
+  assert_equal nil, ('z'..'l').max
+end
+
+assert('Range#min') do
+  # string Range depends on String#upto
+  assert_equal 'f', ('f'..'l').min
+  assert_equal nil, ('z'..'l').min
+end