Generalize the reading of test file data

Currently we have two methods in ganeti.bdev_unittest.py of computing
the test data file name - and, of course, they don't give the same
results.

The patch moves the functions to compute the test file name and reading
of its contents to the GanetiTestCase class in testutils, which allows
running the tests from the command line as well.

We also change assertFileContent to use utils.ReadFile.

Reviewed-by: imsnah