Fix and re-enable Dumps' checkpoint tests

Fix and re-enable Dumps' checkpoint tests

It seems that fread can now bring 0.5MB into a buffer at once. That
recently made the checkpoint tests erratic, as they relied on the
backupTestPass.inc dump reading loop taking at least two rounds, as
only the second round can trigger creating the second checkpoint file.

By taking advantage of the just added parameter to set the buffer
size, we lower the used buffer size for the tests. Thereby, we force
more than one iteration of the dump reading loop, and finally make the
tests pass again reliably.
Hence, the tests get enabled again.

Bug: T70653
Change-Id: I4449b42558ada8bd579a33b651f2a630a47bfe1c