Fixed failing tests in VS 2017 by moving to MSBuild 15 and disabling parallelization

[azeno]

There were several minor issues which prevented the tests from running properly in VS 2017 as well as console:

• Some tests failed complaining that SkipNonexistentTargets is not a valid task property. I first tried building the tests with RestoreUseSkipNonexistentTargets = false but this just led to follow-up errors like OutputPath not set etc. so I tried updating to MSBuild 15 and voila the affected tests passed inside VS but now running all of them other started to fail randomly. So I also had to disable the test parallelization for VS as well as the console runner.
• I've further modified a few tests to invoke the Restore target before doing their actual check.
• Enabled two cross targeting tests and fixed include of targets file

I hope this PR also fixes the build errors we see on appveyor so I can rebase two other PRs on this one to at least get a pre-release package out of it.

[kzu]

Fantastic. Thanks a lot!