Get Non-windows platforms working

[bording]

This includes the changes needed to avoid anything that is platform-specific. Most of the changes are self-explanatory, for example dealing with path separator and line ending differences.

There are a few that could benefit from some more detail:

Marshal.SizeOf

The problem with the StructConventionsTest was that for the LogicalAddress struct, instead of showing a size, it instead had an error like Type cannot be marshaled as an unmanaged structure; no meaningful size or offset can be computed. On windows, calling Marshal.SizeOf on a struct with a reference field works fine, but not on other platforms. I found https://github.com/dotnet/corefx/issues/20442 describing the exact problem, and that it's by design that it doesn't work, because a reference field marshals as IUnknown, which is a windows-specific COM interface. So, on other platforms it throws instead. I adjusted the test to skip the size check if the struct has a reference field and indicate that it was skipped.

On a side note, the use of Marshal.SizeOf isn't really correct for how we're trying to use it in the test. What we really want here is the managed size of the struct, but the CLR specifically doesn't allow you to find that out. So instead, we're determining the unmanaged size of the struct if we were going to marshal it to native code. However, that size isn't guaranteed to be the same as the size in managed memory!

CopyLocalLockFileAssemblies

At first, I ran into a problem where dotnet test could not find any tests, but that was because of the differences in the locations of the NuGet cache between machines and the fact that we can't build on non-windows currently. Setting the CopyLocalLockFileAssemblies property works around this by making sure all the dependencies are in the build output folder instead of needing to be loaded from the cache.

ApprovalTests

The tests that rely on the ApprovalTests library were failing with an error about COM not being supported, but that turned out to be because of our chosen reporters. After switching to the NUnit reporter, I was still seeing failures because ApprovalTests couldn't find the *.approved.txt files. Looking into the ApprovalTests code, it uses some stack-walking code to try and figure out the location of the file. That doesn't seem to be working properly on linux, so the tests fail.

I looked around for another approval tests library, and found https://github.com/droyad/Assent. After building locally to deal with the lack of strong-naming in the official package, I found that I was running into the same problem: it couldn't find the *.approved.txt files. Instead of stack-walking, it uses the CallerFilePath attribute to get the file name, but this path is baked in at compile-time... so since we aren't building on linux, the paths are all wrong and it still can't find them! 😢

Ultimately, I decided to make the tests Assert.Ignore if they're running on a non-windows platform for now. If we can get to a point where we can actually build on linux, then we should take another look at Assent.

---

With these changes, I can now successfully run all tests on linux locally. 🎉

[timbussmann]

can you share some background on the Marshal.SizeOf differences across the platforms?

[bording]

I updated the root comment to include information on the Marshal.SizeOf differences.

[timbussmann]

would it be possible to write an attribute for that check?

[bording]

NUnit has a Platform attribute that I tried to use first, bu it doesn't exist in their current netstandard assemblies. I suspect it's because it relies on the above code behind the scenes, and that API was added back to .NET Core in 2.0.

[bording]

I've also verified that the tests run on macOS as well.