Convert some `vec_type` TLVs to `required_vec`

Convert some `vec_type` TLVs to `required_vec`

This converts some required TLVs to `required_vec` which are, in
fact, required (and have been written forever).

* `HTLCFailReason` hasn't changed since many structs were converted
  to TLVs in 66784e32fe454e9a5b2080b85fc4d8816ac5e436.
* `NodeInfo::channels` has been written since `NetworkGraph`
  structs were converted to TLVs in 321b19c4d96a49b39b5583d0f12fea.
* Several test-only TLV writes were converted.