tests: consolidate the `file_size` function into `test-lib-functions.sh`

tests: consolidate the `file_size` function into `test-lib-functions.sh`

In 8de7eeb54b6 (compression: unify pack.compression configuration
parsing, 2016-11-15), we introduced identical copies of the `file_size`
helper into three test scripts, with the plan to eventually consolidate
them into a single copy.

Let's do that, and adjust the function name to adhere to the `test_*`
naming convention.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Reviewed-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>