test: loosen certain sandbox restrictions when collecting coverage

test: loosen certain sandbox restrictions when collecting coverage

With `ProtectSystem=strict` gcov is unable to write the *.gcda files
with collected coverage. Let's add a yet another switch to make such
restriction less strict to make gcov happy.

This addresses following errors:
```
...
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/binfmt-util.c.gcda:Cannot open
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/base-filesystem.c.gcda:Cannot open
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/barrier.c.gcda:Cannot open
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/ask-password-api.c.gcda:Cannot open
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/apparmor-util.c.gcda:Cannot open
systemd-networkd[272469]: profiling:/systemd-meson-build/src/shared/libsystemd-shared-249.a.p/acpi-fpdt.c.gcda:Cannot open
...
```