amqp10_common: Don't wrap export for test in test macro

The application is not always recompiled which causes tests to fail
because they cannot call `serial_number:usort/1`.

(cherry picked from commit c9504fa)

deps/amqp10_common/src/serial_number.erl

@@ -15,9 +15,8 @@
          diff/2,
          foldl/4]).
 
--ifdef(TEST).
+%% For tests.
 -export([usort/1]).
--endif.
 
 -type serial_number() :: sequence_no().
 -export_type([serial_number/0]).