Most of the test indices are snatched directly from the unit test suite of git itself, usually by running something like the following
./t1700-split-index.sh -r 2 --debug
Then one finds all test state and the index in particular in trash directory/t1700-split-index/.git/index and can possibly copy it over and use as fixture. The preferred way is to find a test of interest, and use its setup code within one of our own fixture scripts that are executed once to generate the file of interest.