Add new workflow for tests that were only running on Travis CI:
- fast test suite (sh, bash, dash, zsh)
- installation_iojs without curl (sh, bash, dash, zsh)
- installation_node with and without curl (sh, bash, dash, zsh)
- xenial test suite (sh, bash, dash, zsh)
The installation_node and xenial tests run on ubuntu-20.04 to match
the Travis xenial environment as closely as possible.
