Increase parallel_tests to 8 on FreeBSD workers

[vstinner]

The Python test runner reports "CPU count: 8", so increase the concurrency.

[vstinner]

Oh wow, the Refleaks buildbot now only takes 35min 21sec instead of 1h 23min!

• Before: https://buildbot.python.org/#/builders/1613/builds/3936
• After: https://buildbot.python.org/#/builders/1613/builds/3943

[hugovk]

Could update a couple of others:

Under (configured < CPU count)

Worker	Configured	CPU count	Notes
diegorusso-aarch64-bigmem	8	175	Bigmem tests, shared with benchmarks. Intentional.
kulikjak-solaris-sparcv9	16	512	SPARC hardware threads. Owner's choice in #316.
billenstein-macos	default 2	8	Never set. Clear win.
pablogsal-macos-m1	4	8	Set in 2022. Clear win.
itamaro-centos-aws	10 (×2 builders)	16	Two builds can overlap already. Marginal.
itamaro-macos-intel-aws	10	12	Marginal.
malvex-nixos-x86_64	10	12	Marginal.
bcannon-wasi	2 (×2 builders)	4	Effectively 4. Fine.
skumaran-ubuntu-x86_64	1	2	Reduced on purpose in #801. Leave.

Over (configured > CPU count)

Worker	Configured	CPU count
cstratak-CentOS10-aarch64	32	16
cstratak-CentOS9-aarch64	32	16
cstratak-fedora-rawhide-aarch64	32	16
cstratak-fedora-stable-aarch64	32	16
cstratak-RHEL8-aarch64	32	16
itamaro-win64-srv-22-aws	20	16
itamaro-macos-arm64-aws	10	8
cstratak-c10s-s390x	10	8
cstratak-CentOS9-ppc64le	10	8
cstratak-fedora-rawhide-ppc64le	10	8
cstratak-fedora-rawhide-s390x	10	8
cstratak-fedora-rawhide-x86_64	10	8
cstratak-fedora-stable-ppc64le	10	8
cstratak-fedora-stable-s390x	10	8 (offline)
cstratak-fedora-stable-x86_
cstratak-RHEL8-ppc64le	10	8
cstratak-RHEL8-x86_64	10	8
cstratak-CentOS10-fips-x86_64	6	4
cstratak-CentOS9-fips-x86_64	6	4
cstratak-RHEL8-fips-x86_64	6	4

Matched

Worker	Configured	CPU count
opsec-fbsd14	8	8
opsec-fbsd15	8	8
opsec-fbsd16	8	8
ware-win11-arm64	8	8
ware-alpine	6	6
ware-debian-x86	6	6
bolen-windows10	default 4	4
gps-raspbian	4	4
onder-riscv64	4	4
rise-riscv64-2	4	4
rise-riscv64-3	4	4
rise-riscv64-4	4	4
savannah-raspbian	4	4
stan-aarch64-ubuntu	4	4
stan-raspbian	4	4
ware-ws2025	4	4
pablogsal-arch-x86_64	default 2	2
ware-win11	2	2

No data: kushaldas-wasi (no recent build reached the test step) and pablogsal-rasp (offline).