Add integration tests as part of Github's CI

[facundobatista]

They should run fades and check that it worked ok, probably running a script that needs one or two dependencies and validating the output.

Cases to check:

• requesting a venv with the same Python and with another one
• with and without system-site-packages

Where to run:

• MacOS
• Windows
• Ubuntu
• Other linuxes

This is a pre-requisite for #411 .

[facundobatista]

Implemented most of this in #427.

It's missing to activate some of the integration tests (marked with XXX, to work that out while working in #411), and left one of the tests out in Arch because the platform was complicated.

[facundobatista]

Done now