split-index: disable the fsmonitor extension when running the split index test

The split index test t1700-split-index.sh has hard coded SHA values for the index. Currently it supports index V4 and V3 but assumes there are no index extensions loaded. When manually forcing the fsmonitor extension to be turned on when running the test suite, the SHA values no longer match which causes the test to fail. The potential matrix of index extensions and index versions can is quite large so instead temporarily disable the extension before attempting to run the test until the underlying problem of hard coded SHA values is fixed. Signed-off-by: Ben Peart <benpeart@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>