Add NEAREST filtered blitting tests

Add NEAREST filtered blitting tests

This fixes a couple of issues with nearest filtered blitting tests
and adds them back to the mustpass.

- Corrected test case names (esp. for D/S formats)
- Modified common code used by linear cases too (shouldn't have
  an observable effect on tests)
- Used tcuTexLookupVerifier to sample source image with an offset
  that accounts for rounding errors in coordinate calculation
- Enabled nearest cases in mustpass

New tests:
- dEQP-VK.api.copy_and_blit.blit_image.all_formats.*_nearest

Modified tests:
- dEQP-VK.api.copy_and_blit.blit_image.all_formats.*

Fixes #483, #565

Change-Id: I176c2b50a656268a31f1f35fef1ff90647039727