Gifti module unit tests

tests/test_gifti.py

tests/testdata/example.shape.gii

tests/testdata/example.surf.gii

tests/testdata/example4D.shape.gii

tests/testdata/example4D_multiple_darrays.shape.gii

tests/testdata/example_bad1.surf.gii

tests/testdata/example_bad2.surf.gii

tests/testdata/example_bad3.surf.gii

tests/testdata/example_bad4.surf.gii

tests/testdata/example_bad5.surf.gii

tests/testdata/example_bad6.surf.gii