tests: create a separate module for helper functions

tests: create a separate module for helper functions

I am going to add a test for the Fiemap module, and it will need the
'create_random_sparse_file()' function from the base API test. Therefor, move
this function to a separate 'test_helpers.py' module which will be shared
across tests.

Additionally, remove the holes verification from 'create_random_sparse_file()'
- the test which I will add soon verifies holes separately.

Also change the egging to exclude the tests.

Change-Id: I2c81251c5e1884a4f644e89b417c8a18050aeada
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>