Buffering tests should have cases for BytesIO/StringIO variants that return less data on read()

[smheidrich]

I.e. instead of BytesIO(b"abc").read() returning b"abc", it returns only b"a", requiring multiple calls to get the full buffer. This is allowed, cf. https://docs.python.org/3.14/library/io.html#io.RawIOBase.read.

Follow-up on #105

Related to #107