Add command-line option to generate XML test report.

[teo-tsirpanis]

This PR:

• Adds two command-line options --nunit-summary and --junit-summary that accept a string parameter and export a test summary in a respectively formatted XML file. It avoids incidents like #373 by making the feature more accessible known.
• Removes the Expecto.TestResults project and moves its content to the main Expecto project. Usually, developers create a different package for the same library when it depends on something the main package shouldn't. Unlike Expecto.FsCheck or Expecto.Hopac, Expecto.TestResults has no such special dependencies and therefore should have been part of Expecto proper. This change allows for the former bullet point to be easily implemented.
• Splits the huge Expecto.fs file into three smaller ones: Expecto.Impl.fs which contains the Impl module, Test.fs which contains the FlatTest type and the Test module, and keeps the rest to Expecto.fs. Such a big file was hard-to-maintain, and splitting it was also necessary to implement the first bullet point.

The removal of the Expecto.TestResults assembly might be a breaking change, but I believe its introduction is totally justifiable, given how easy it will become to generate test reports without code changes.

[haf]

Well, it all looks great, like always when you're hacking away.

[haf]

Did you manage to keep all public surface areas the same?

[teo-tsirpanis]

Did you manage to keep all public surface areas the same?

Yes, I did. The only thing I removed was a misspelled identifier named depricated (had no reason to exist, and it had already been marked as obsolete).

Given that this change is going to be pushed for a major version bump, I am going to entirely remove the existing API to generate test results; it is already exposed via the more uniform CLIArgs.

[haf]

I am going to entirely remove the existing API to generate test results

Ok, so this is WIP still?

[teo-tsirpanis]

Yes.

[teo-tsirpanis]

It's ready.

[haf]

@teo-tsirpanis thank you! I'll try release tonight

[teo-tsirpanis]

Good, but a new major version? That fast? I think a minor version increase (8.14.0) is more appropriate. Unless you don't have any reservation with major version releases (like me).

And don't forget to mark the Expecto.TestResults package as deprecated.

If you make it a major version increase eventually, it would be better to wait as I am working to fix #328 (which is also tagged with semver-MAJOR).

[haf]

Sure, I have no problem delaying, but part of the contract with IDEs and users is that we don't break even the slightest thing in between minors :)