Run XM tests with older system monos

[rolfbjarne]

Running an XM app using system mono usually works if the system mono is older than the one used at build time.

Unfortunately right now we have no way of knowing which older system mono's might work.

We should add tests (maybe reusing those running on older macOS versions) and run them with older system monos.

And then if we find that a particular mono version is required, we should bump the dependency as described here: #4120.

Reference: https://xamarinhq.slack.com/archives/C07MKNX1T/p1527156257000264
Reference: #4116

[rolfbjarne]

Added POC (rolfbjarne@64fa71c), but it will conflict with changes in the xcode10 branch, so we can wait until xcode10 has been merged into master before continuing.