numeric/accumulator test: fix compilation warning

http://my.cdash.org/viewBuildError.php?type=1&buildid=475786

.../test/unit/numeric/accumulator.cpp:53:42: warning: comparison between
signed and unsigned integer expressions [-Wsign-compare]

I used the opportunity to prefer pre-increment (++i) over post-increment
(i++) which is the general rule (although it does not make a difference
for POD types).