Relocate test project: test/yavscTests -> src/Yavsc.Org.Tests

Move the integration test project from the top-level test/ directory into
src/ alongside the projects it tests. Rename the project (and folder) to
Yavsc.Org.Tests to match .NET conventions and reflect that it tests the
Org runtime primarily.

Path changes:
- test/yavscTests/yavscTests.csproj -> src/Yavsc.Org.Tests/Yavsc.Org.Tests.csproj
- All .cs / .json / .resx files moved to their new location
- PostItViewModelTests moved out to the dedicated src/PostIt.Tests project
  (it was unrelated to Org testing)

Build adjustments:
- <ProjectReference> paths shortened (..\..\src\X -> ..\X)
- PostIt project reference removed (covered by its own test project)
- <OutputType>exe added (required by xunit.v3)
- xunit.v3.common and xunit.v3.extensibility.core added to package versions

Solution + sln:
- yavsc.sln Project Name updated to 'Yavsc.Org.Tests' and path updated
- GUID preserved so existing build configs stay valid

Static web assets:
- The CopyStaticWebAssetsManifest target was hard-coding the destination
  filename to 'testhost.staticwebassets.endpoints.json', which worked
  when the assembly was named 'yavscTests'. Now that the assembly name
  is 'Yavsc.Org.Tests', ASP.NET Core's MapStaticAssets() looks for
  'Yavsc.Org.Tests.staticwebassets.endpoints.json' (entry-assembly-based
  resolution). Use $(MSBuildProjectName) so the copy target stays
  correct under any future rename.

src/Yavsc.Org.Tests/NonRegression/AbstractTests.cs

@@ -0,0 +1,27 @@
+
+using Yavsc.Server.Helpers;
+
+namespace yavscTests
+{
+    [Collection("Yavsc Abstract tests")]
+    [Trait("regression", "II")]
+    public class AbstractTests
+    {
+        readonly ITestOutputHelper output;
+        public AbstractTests(ITestOutputHelper output)
+        {
+            this.output = output;
+        }
+        [Fact]
+        public void UniqueFilenameAfterCleaning()
+        {
+            var name1 = "content:///scanned_files/2020-06-02/00.11.02.JPG";
+            var name2 = "content:///scanned_files/2020-06-02/00.11.03.JPG";
+            var cleanName1 = AbstractFileSystemHelpers.FilterFileName(name1);
+            var cleanName2 = AbstractFileSystemHelpers.FilterFileName(name2);
+            output.WriteLine($"{name1} => {cleanName1}");
+            output.WriteLine($"{name2} => {cleanName2}");
+            Assert.True(cleanName1 != cleanName2);
+        }
+    }
+}