This commit adds the packs and queries from the actions input to the
config file used by the CodeQL CLI.
When the `+` is used, the actions input value is combined with the
config value and when it is not used, the input value overrides the
config value.
This commit also adds a bunch of integration tests for this feature.
In order to avoid adding too many new jobs, all of the tests are
run sequentially in a single job (matrixed across relevant operating
systems and OSes).
