For one, this allows us to use `require` instead of `assert` and
`continue`, which was awkward to read. That works because it's ok to
fail a subtest: the remaining subtests are executed after it.
Also, this shows which test was executed, making debugging easier:
=== RUN TestCrossArchDepsolve
=== RUN TestCrossArchDepsolve/fedora-30
=== RUN TestCrossArchDepsolve/fedora-30/x86_64
=== RUN TestCrossArchDepsolve/fedora-30/x86_64/ami
...
You can now also run those sub tests in isolation:
osbuild-dnf-json-tests -test.run TestCrossArchDepsolve/fedora-30/x86_64/ami
Lastly, it enables running those tests in parallel (not part of this
patch) by calling `t.Parallel()`.
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