bloom: add test helper to return murmur3 hash

bloom: add test helper to return murmur3 hash

In bloom.h, murmur3_seeded_v2() is exported for the use of test murmur3
hash. To clarify that murmur3_seeded_v2() is exported solely for testing
purposes, a new helper function test_murmur3_seeded() was added instead
of exporting murmur3_seeded_v2() directly.

Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
Signed-off-by: Junio C Hamano <gitster@pobox.com>