t/sharded/index.t: random failures

[eserte]

With the same perl configurations t/sharded/index.t sometimes passes and sometimes not. A failure looks like this:

    #
    # Failed test 'exists()'
    # at /usr/perl5.20.1p/lib/site_perl/5.20.1/Metabase/Test/Index.pm line 202.
    # Caught exception in subtest: Can't call method "search" on an undefined value at /home/cpansand/.cpan/build/2017032118/Metabase-Backend-SQL-1.001-15/blib/lib/Metabase/Index/SQLite/Sharded.pm line 92.
#   
# Failed test 'Run index tests on Metabase::Index::SQLite::Sharded'
# at /usr/perl5.20.1p/lib/site_perl/5.20.1/Test/Routine/Runner.pm line 115.
# Looks like you failed 1 test of 1.
t/sharded/index.t ........
Dubious, test returned 1 (wstat 256, 0x100)
Failed 1/1 subtests