build-sys: disable test-cgroup-mask, it mangles valid user data in /run/user/$UID/

build-sys: disable test-cgroup-mask, it mangles valid user data in /run/user/$UID/

  $ ./test-cgroup-mask
  ...
  rmdir("/run/user/2702/systemd/generator") = 0
  open("/run/user/2702/systemd/generator.late", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_NOFOLLOW|O_NOATIME|O_CLOEXEC) = 5
  fstat(5, {st_mode=S_IFDIR|0755, st_size=3200, ...}) = 0
  fcntl(5, F_GETFL)                       = 0x78800 (flags O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|O_NOFOLLOW|O_NOATIME)
  fcntl(5, F_SETFD, FD_CLOEXEC)           = 0
  getdents(5, /* 160 entries */, 32768)   = 10072
  unlinkat(5, "org.gnome.Weather.Application.busname", 0) = 0
  unlinkat(5, "dbus-org.gnome.Weather.Application.service", 0) = 0
  ...