[One .NET] fix for incremental CoreCompile

[jonathanpeppers]

Context: https://github.com/xamarin/Xamarin.Forms/tree/main-handler

I noticed that when building Maui, CoreCompile seems to be running
on every build no matter what:

Building target "CoreCompile" completely.
Input file "obj\Debug\net6.0-android\Core-net6.csproj.CoreCompileInputs.cache" is newer than output file "bin\Debug\net6.0-android\Microsoft.Maui.xml".

I could reproduce this in a test:

• Build AppA & LibraryB
• Build AppA & LibraryB again, LibraryB will run CoreCompile
  every time.

There is a _GenerateCompileDependencyCache target that basically does:

<Hash ItemsToHash="@(CoreCompileCache)">
  <Output TaskParameter="HashResult" PropertyName="CoreCompileDependencyHash" />
</Hash>
<WriteLinesToFile
    Lines="$(CoreCompileDependencyHash)"
    File="$(IntermediateOutputPath)$(MSBuildProjectFile).CoreCompileInputs.cache"
    Overwrite="True"
    WriteOnlyWhenDifferent="True"
/>

https://github.com/dotnet/msbuild/blob/83cd7d4e36b71d5b2cefd02cb9a5a58d27dd6a75/src/Tasks/Microsoft.Common.CurrentVersion.targets#L3529

This *.CoreCompileInputs.cache file triggers CoreCompile to run
again when it needs to.

However, this file is actually updating on every build, because:

1. Our "outer" build has all our preprocessor defines listed in
   @(CoreCompileCache) like __MOBILE__, __ANDROID__, etc.
2. The "inner" build for each $(RuntimeIdentifier) does not have
   these symbols!

And so we get into a situation where CoreCompile will always run.
The inner & outer builds write different values in this file.

To solve this problem, I added our _AddAndroidDefines to run before
CoreCompile in inner builds.

I also needed some changes to our MSBuild test framework:

• Make IsTargetSkipped() and AssertTargetIsSkipped() supported for
  new project types.
• Make IsTargetSkipped() return false if a Building target "{target}" completely. message is found.