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So far BigFile was not unit-tested and because of recent BigFile patches
and fixes Romain suggested to write tests for it.

We test:

    - working with BigFile via its public interface:

      * GET/PUT both in plain and with ranges variants,
      *.getData()/.getSize(), and
      * recently-introduced ._appendData()

    - that BigFile correctly handles situations where .data is either
      None or str or BTreeData and that migration automatically happens
      to BTreeData on append.

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Unlike common case, BigFile more directly works on REQUEST and RESPONSE
(instead of plain object publishing), so to test it we need not only call
methods and compare return values but first prepare proper
request/response, set them up and analyze response headers and content
after method invocation happened.

For preparing request/response Zope provides utility
Testing.makerequest() and its 2 variations but for our case they all
turned out to be not flexible enough - e.g. Testing.makerequest()...