tests: disable GIT_TEST_SPLIT_INDEX for sparse index tests

tests: disable GIT_TEST_SPLIT_INDEX for sparse index tests

The sparse index and split index features are said to be currently
incompatible [1], and consequently GIT_TEST_SPLIT_INDEX=1 might
interfere with the test cases exercising the sparse index feature.
Therefore GIT_TEST_SPLIT_INDEX is already explicitly disabled for the
whole of 't1092-sparse-checkout-compatibility.sh'.  There are,
however, two other test cases exercising sparse index, namely
'sparse-index enabled and disabled' in
't1091-sparse-checkout-builtin.sh' and 'status succeeds with sparse
index' in 't7519-status-fsmonitor.sh', and these two could fail with
GIT_TEST_SPLIT_INDEX=1 as well [2].

Unset GIT_TEST_SPLIT_INDEX and disable the split index in these two
test cases to avoid such interference.

Note that this is the minimal change to merely avoid failures when
these test cases are run with GIT_TEST_SPLIT_INDEX=1.  Interestingly,
though, without these changes the 'git sparse-checkout init --cone
--sparse-index' commands still succeed even with split index, and set
all the necessary configuration variables and create the initial
'$GIT_DIR/info/sparse-checkout' file, but the test failures are caused
by later sanity checks finding that the index is not in fact a sparse
index.  This indicates that 'git sparse-checkout init --sparse-index'
lacks some error checking and its tests lack coverage related to split
index, but fixing those issues is beyond the scope of this patch
series.

[1] https://public-inbox.org/git/48e9c3d6-407a-1843-2d91-22112410e3f8@gmail.com/

[2] Neither of these test cases fail at the moment, because
    GIT_TEST_SPLIT_INDEX=1 is broken and never splits the index, and
    it broke long before the sparse index feature was added.
    This patch series is about to fix GIT_TEST_SPLIT_INDEX, and then
    both test cases mentioned above would fail.

(The diff is best viewed with '--ignore-all-space')

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>