Extract IO::Buffer.for string locking test (#8729)

String locking with locktmp is not really part of the public API,
and the test relies in a side effect of using it to protect the
buffer. On other implementations without locktmp this does not
fail. Separate into its own test so it can be excluded from public
API expectations.

test/ruby/test_io_buffer.rb

@@ -102,11 +102,6 @@ class TestIOBuffer < Test::Unit::TestCase
     IO::Buffer.for(string) do |buffer|
       refute buffer.readonly?
 
-      # Cannot modify string as it's locked by the buffer:
-      assert_raise RuntimeError do
-        string[0] = "h"
-      end
-
       buffer.set_value(:U8, 0, "h".ord)
 
       # Buffer releases it's ownership of the string:
@@ -116,6 +111,16 @@ class TestIOBuffer < Test::Unit::TestCase
     end
   end
 
+  def test_string_mapped_buffer_locked
+    string = "Hello World"
+    IO::Buffer.for(string) do |buffer|
+      # Cannot modify string as it's locked by the buffer:
+      assert_raise RuntimeError do
+        string[0] = "h"
+      end
+    end
+  end
+
   def test_non_string
     not_string = Object.new