Add ferretdb to test matrix

[majst01]

They claim to be 100% compatible with MongoDB so prove that and see the differences

https://github.com/FerretDB/FerretDB

[majst01]

Does not work yet because:

prefix_test.go:152: Could not create prefix: unable to insert prefix:192.168.1.0/24, error:write exception: write errors: [invalid key: "192.168.1.0" (key must not contain '.' sign)]

This is because this is a known incompatibility https://docs.ferretdb.io/diff/

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Document restrictions:
document keys must not contain . sign;