t3008: skip lazy-init test on a single-core box

t3008: skip lazy-init test on a single-core box

The lazy-init codepath will not be exercised uniless threaded.  Skip
the entire test on a single-core box.  Also replace a hard-coded
constant of 2000 (number of cache entries to manifacture for tests)
with a variable with a human readable name.

Signed-off-by: Kevin Willford <kewillf@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>