[3.12] gh-110184: Fix subprocess test_pipesize_default() (GH-110465) (#110471)

gh-110184: Fix subprocess test_pipesize_default() (GH-110465)

For proc.stdin, get the size of the read end of the test pipe.

Use subprocess context manager ("with proc:").
(cherry picked from commit d023d4166b255023dac448305270350030101481)

Co-authored-by: Victor Stinner <vstinner@python.org>