feat(jobspec): add iterative reassembly tests

+ Only to some 3node testbeds (alt, icx, snr and tsh).
+ Both MRR and NDRPDR.
+ 1518B only. All core counts.
+ Primary NIC only. Drv_vfio_pci only.
+ Only two suites:
 + ethip4ipsec1000tnlsw-ip4base-int-aes256gcm-reassembly
 + ethip4gtpusw-ip4base-reassembly
  - This is expected to fail frequently due to VPP-2077.
+ Added to the end of crypto or ip4-tunnels section,
  as the focus is on fragmentation and reassembly, not on encap/decap.
- Coverage not added here.

Change-Id: Ifca2a13e43b9d3b7ddc0eaa8fd6d3e1a56b90c7f
Signed-off-by: Vratko Polak <vrpolak@cisco.com>