I suspect it is not possible, but maybe I'm lucky:
how can I create a mock object with MiniTest that expects a
call to the to_s method?

1.9.3-p194 :001 > require 'minitest/mock'
...
1.9.3-p194 :002 > obj = MiniTest::Mock.new
...
1.9.3-p194 :003 > obj.expect(:size, 5)
...
1.9.3-p194 :004 > obj.expect(:to_s, 'an object')
...
1.9.3-p194 :005 > obj.size
  => 5
1.9.3-p194 :006 > obj.to_s
  => "#<MiniTest::Mock:0x82eea98>"


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