support/testing: add openssh runtime test

This new runtime test is based on test_dropbear.py. The only required change
is to use "-oStrictHostKeyChecking=no" instead of "-y" to accept the new key.

Since the base test infra only provide a uClibc-ng toolchain, add a second
test using a glibc based internal toolchain.

For example, this allow to trigger the openssh 8.1p bug with glibc 2.31 [1].

[1] https://bugs.archlinux.org/task/65386



Signed-off-by: Romain Naour <romain.naour@smile.fr>
yann.morin.1998@free.fr:
  - deduplicate the whole test
  - don't provide any NIC, we only need and use lo
  - simplify post-build script (append with cat, don't munge with sed)
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>