Move regression test as String#dump is not in ther core but the mrbgems.

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

@@ -14,6 +14,7 @@ assert('String#getbyte') do
 end
 
 assert('String#dump') do
+  ("\1" * 100).dump     # should not raise an exception - regress #1210
   "foo".dump == "\"foo\""
 end
 

test/t/string.rb

@@ -415,10 +415,6 @@ assert('String#each_byte') do
   bytes1 == bytes2
 end
 
-assert('String#dump') do
-  ("\1" * 100).dump     # should not raise an exception - regress #1210
-end
-
 assert('String#inspect') do
   ("\1" * 100).inspect  # should not raise an exception - regress #1210
 end