t/perf: use 'test_file_size' in more places

t/perf: use 'test_file_size' in more places

The perf test suite prefers to use test_file_size over 'wc -c' when
inside of a test_size block. One advantage is that accidentally writign
"wc -c file" (instead of "wc -c <file") does not inadvertently break the
tests (since the former will include the filename in the output of wc).

Both of the two uses of test_size use "wc -c", but let's convert those
to the more conventional test_file_size helper instead.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>