Adding headers to test common header files

[guimafelipe]

This pull request updates several test header files to improve consistency and ensure all necessary dependencies are explicitly included. The main focus is on adding missing system and library includes, which helps prevent build issues and makes dependencies clearer for future maintenance.

Dependency inclusion improvements:

• Added missing includes such as <windows.h>, <winrt/base.h>, <WexTestClass.h>, <appmodel.h>, <Verify.h>, and <filesystem> to multiple test headers including TestDef.h, WindowsAppRuntime.Test.AppModel.h, WindowsAppRuntime.Test.Bootstrap.h, WindowsAppRuntime.Test.FileSystem.h, WindowsAppRuntime.Test.Package.h, WindowsAppRuntime.Test.TAEF.cppwinrt.h, and WindowsAppRuntime.Test.TAEF.h. [1] [2] [3] [4] [5] [6] [7]

These changes help ensuring that each header is independent and handle its own dependencies. Without these explicit dependencies, when creating a new test file, we need to know precisely the order in which to include each header for them to work, as some headers are dependent on external includes to exist. Or use one of the .pch files as example, but then making it harder to maintain as the dependencies are hidden and we end up including unnecessary files.
This is also stated on C++ Core Guidelines.

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[DrusTheAxe]

I don't follow the reason for most of these changes. See comment for more detail

[DrusTheAxe]

Why?

Is this currently a problem or is this a stylistic change?

This is the most obvious example - why does this need to include windows.h? That's routinely the 1st include in any precompiled header, and in any source file in projects not using precompiled headers. Also, some include <unknown.h> rather than windows.h (the former includes the latter, plus other things).

I don't see the value including windows.h in any header file (except precomp.h/pch.h)

[guimafelipe]

The current problem is that for some of these headers, you can't include them alone without including these dependencies before in your file. Creating a new test that uses one of these headers requires knowing what they depend on, and it is not possible to just include the header that you actually want to use alone, requiring the include of windows.h before including the desire header for example.
I think it is a good practice to have headers that work as their own (by being possible to compile a .cpp file including only it, which is what I mean when I say a header can be compiled) and it would make it easier for others to use them within the project.

For this specific case, this header can be compile on its own because it includes windows.h within appmodel.h, but it is hidden under a conditional macro. So I included it here to make it safer.

Do you think there are any downsides on doing this change? We can close this PR if it is not desired.