Test SecureRandom.uuid format

SecureRandom uses v4 UUIDs, which are completely random except for
6 bits, 4 in the version field and 2 in the clk_seq_hi_res field.
Add a test that those bit patterns are set correctly for v4 UUIDs,
per RFC 4122 section 4.4.

Fixes [Bug #13603]

test/test_securerandom.rb

@@ -140,6 +140,11 @@ end
   def test_uuid
     uuid = @it.uuid
     assert_equal(36, uuid.size)
+
+    # Check time_hi_and_version and clock_seq_hi_res bits (RFC 4122 4.4)
+    assert_equal('4', uuid[14])
+    assert_include(%w'8 9 a b', uuid[19])
+
     assert_match(/\A\h{8}-\h{4}-\h{4}-\h{4}-\h{12}\z/, uuid)
   end