Use "make test-p" in manylinux build

[radarhere]

manylinux_2_28 has started failing when running .ci/test.sh - https://github.com/python-pillow/docker-images/actions/runs/34656097159/job/103448735813

ERROR: usage: python -m pytest [options] [file_or_dir] [file_or_dir] [...]
python -m pytest: error: unrecognized arguments: --numprocesses=logical --dist=worksteal
inifile: /Pillow/pyproject.toml
rootdir: /Pillow

make: *** [Makefile:36: test] Error 4

This is a result of python-pillow/Pillow#9945 requiring pytest-xdist.

If we replace .ci/test.sh with make test-p, then pytest-xdist will be installed.