Signed model assertions for use in the spread tests can be generated using
gendeveloper1model tool. The assertion is signed by developer1 key, which is
built-into the snapd binary used during the tests.
To build the tool:
$ export GO111MODULE=off
$ go install ./tests/lib/gendeveloper1model
Generating the assertions is done like this:
$ cd tests/lib/assertions
$ gendeveloper1model < developer1-pc-18.model.json > developer1-pc-18.model
