test: main loop from blitters-test added as a new function to utils.c

test: main loop from blitters-test added as a new function to utils.c

This new generalized function can be reused in both blitters-test
and scaling-test. Final checksum calculation changed in order to make
it parallelizable (it is a sum of individual 32-bit values returned
by a callback function, which is now responsible for running test-specific
code). Return values may be crc32, some other hash or even just zero on
success and non-zero on error (in this case, the expected result of the
whole test run should be 0).