support/testing: test_systemd.py: add linux fragment to enable CONFIG_BINFMT_MISC

While investigating [1] one units failed due to missing kernel option
CONFIG_BINFMT_MISC needed by "proc-sys-fs-binfmt_misc.mount" service.

It's because the kernel support autofs4 but not MISC binaries.

Since the systemd test infra use the default defconfig (vexpress),
we need to provide a linux fragment to enable CONFIG_BINFMT_MISC.

[1] https://gitlab.com/buildroot.org/buildroot/-/jobs/454255917



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr:
  - move the kernel config with the others in conf/
]
Tested-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>