Support loading precompiled NEF files in neotest

[Chubru]

Is your feature request related to a problem? Please describe.

At the moment neotest allows contract compilation from source code. However, in some cases, it would be useful to load a contract from a NEF file. This can be especially helpful, for instance, when testing contract migration from a previous version.

Describe the solution you'd like

Add a method to allow loading contracts from a NEF file.

[roman-khimov]

You can preload them in a way similar to how NeoFS contract updates are tested, see https://github.com/nspcc-dev/neofs-contract/blob/master/contracts/netmap/migration_test.go and https://github.com/nspcc-dev/neofs-contract/blob/69e9f737c91fc862dcf88496d8a8f9ebc1f4518a/tests/migration/storage.go#L78, but I agree it's not very elegant.