Add option to PULSE test to create merge.txt file on bc_open()
Fix PULSE counterexample by filtering error messages.

    C1 = [[{set,{var,6},
           {call,bitcask_pulse,fork,
                 [[{init,{state,undefined,false,false,[]}},
                   {set,{not_var,7},
                        {not_call,bitcask_pulse,bc_open,
                                  [false,{true,{231,280,114},1}]}},
                   {set,{not_var,10},
                        {not_call,bitcask_pulse,bc_close,[{not_var,7}]}},
                   {set,{not_var,11},
                        {not_call,bitcask_pulse,bc_open,
                                  [false,{true,{170,447,173},1}]}},
                   {set,{not_var,13},
                        {not_call,bitcask_pulse,bc_close,[{not_var,11}]}},
                   {set,{not_var,15},
                        {not_call,bitcask_pulse,bc_open,
                                  [false,{true,{258,18,280},1}]}}]]}},
      {set,{var,7},
           {call,bitcask_pulse,fork,
                 [[{init,{state,undefined,false,false,[]}},
                   {set,{not_var,2},
                        {not_call,bitcask_pulse,bc_open,
                                  [false,{true,{246,472,469},1}]}},
                   {set,{not_var,3},
                        {not_call,bitcask_pulse,bc_close,[{not_var,2}]}},
                   {set,{not_var,6},
                        {not_call,bitcask_pulse,bc_open,
                                  [false,{true,{73,166,183},1}]}},
                   {set,{not_var,7},
                        {not_call,bitcask_pulse,fold,[{not_var,6}]}}]]}},
      {set,{var,8},
           {call,bitcask_pulse,bc_open,[true,{true,{424,237,233},1}]}},
      {set,{var,10},{call,bitcask_pulse,needs_merge,[{var,8}]}},
      {set,{var,11},{call,bitcask_pulse,merge,[{var,8}]}},
      {set,{var,12},{call,bitcask_pulse,delete,[{var,8},1]}},
      {set,{var,13},{call,bitcask_pulse,merge,[{var,8}]}}],
     {62806,53384,84006},
     [{errors,[]}]].
    eqc:check(bitcask_pulse:prop_pulse(), C1).

The error coming from error_logger & failing the property:

    errors: failed
    [{<0.23310.2>,"Invalid merge input file ~s, deleting : ~p",
      ["1399-272921-941361/merge.txt",enoent]}] /= []

This error message is not fatal, so ignore it.
Fix PULSE counterexample when merge's ReadableFiles is the empty list

    C2 = [[{set,{var,2},
           {call,bitcask_pulse,bc_open,[true,{true,{442,453,173},1}]}},
      {set,{var,3},{call,bitcask_pulse,delete,[{var,2},1]}},
      {set,{var,5},{call,bitcask_pulse,bc_close,[{var,2}]}},
      {set,{var,8},
           {call,bitcask_pulse,bc_open,
                 [true,{true,{172,181,201},100}]}},
      {set,{var,9},{call,bitcask_pulse,fork_merge,[{var,8}]}},
      {set,{var,11},{call,bitcask_pulse,fork_merge,[{var,8}]}}],
     {41544,86022,14862},
     [{errors,[]}]].
    eqc:check(bitcask_pulse:prop_pulse(), C2).

The error from PULSE:

    errors: failed
    [{<0.27136.3>,"Failed to merge ~p: ~p\n",
      [{["1399-274027-165496/1.bitcask.data"],[]},
       {function_clause,
           [{lists,min,[[]],[{file,"lists.erl"},{line,300}]},
            {bitcask,merge1,4,[{file,"src/bitcask.erl"},{line,611}]},
            {bitcask_merge_worker,do_merge,1,
                [{file,"src/bitcask_merge_worker.erl"},{line,195}]}]}]}] /= []