ceph-objectstore-test: fix warning in collect_metadata test

In file included from test/objectstore/store_test.cc:33:0:
../src/gtest/include/gtest/gtest.h: In function ‘testing::AssertionResult testing::internal::CmpHelperNE(const char*, const char*, const T1&, const T2&) [with T1 = long unsigned int, T2 = int]’:
test/objectstore/store_test.cc:82:5: instantiated from here
warning: ../src/gtest/include/gtest/gtest.h:1379:1: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Sage Weil <sage@inktank.com>