Refactor ogr_index_test.py to remove dependency on test order

[DFEvans]

What does this PR do?

Refactor ogr_index_test.py to remove dependency on test order

In particular, incorrect test ordering causes a segfault in some circumstances.

(If interested in diagnosing that further, running pytest ogr/ogr_index_test.py --random-order-seed=8 on commit 364c56b produces it reproducibly for me)

Also split some assertions into separate tests.

What are related issues/pull requests?

#4407

Tasklist

• Refactor test cases
• Review
• Adjust for comments
• All CI builds and checks have passed

[rouault]

 autotest/ogr/ogr_index_test.py:32:1: F401 'gdaltest' imported but unused
autotest/ogr/ogr_index_test.py:124:9: F841 local variable 's_ds' is assigned to but never used
autotest/ogr/ogr_index_test.py:205:9: F841 local variable 's_lyr' is assigned to but never used
autotest/ogr/ogr_index_test.py:240:5: F841 local variable 'expect' is assigned to but never used
autotest/ogr/ogr_index_test.py:244:9: F841 local variable 's_lyr' is assigned to but never used
autotest/ogr/ogr_index_test.py:268:5: F841 local variable 'expect' is assigned to but never used
autotest/ogr/ogr_index_test.py:272:9: F841 local variable 's_lyr' is assigned to but never used
autotest/ogr/ogr_index_test.py:293:5: F841 local variable 'expect' is assigned to but never used
autotest/ogr/ogr_index_test.py:297:9: F841 local variable 's_lyr' is assigned to but never used