Fix assembly resolution exception that would happen during every test

The reason this issue started when the tests were moved upstream was due to a behavior change in the newer nunit.  nunit/nunit#1072

This PR uses more explicit resolution for the input & output which is more correct anyways

linker/Tests/Mono.Linker.Tests.Core/DefaultChecker.cs

@@ -35,13 +35,13 @@ public override void Check(LinkedTestCaseResult linkResult)
 
 			int expectedNumberOfAssertionsToMake = 0;
 
-			using (var original = AssemblyDefinition.ReadAssembly(linkResult.InputAssemblyPath.ToString()))
+			using (var original = ReadAssembly(linkResult.InputAssemblyPath))
 			{
-				expectedNumberOfAssertionsToMake += PerformOutputAssemblyChecks(original, linkResult.LinkedAssemblyPath.Parent);
+				expectedNumberOfAssertionsToMake += PerformOutputAssemblyChecks(original.Definition, linkResult.LinkedAssemblyPath.Parent);
 
-				using (var linked = AssemblyDefinition.ReadAssembly(linkResult.LinkedAssemblyPath.ToString()))
+				using (var linked = ReadAssembly(linkResult.LinkedAssemblyPath))
 				{
-					expectedNumberOfAssertionsToMake += CompareAssemblies(original, linked);
+					expectedNumberOfAssertionsToMake += CompareAssemblies(original.Definition, linked.Definition);
 				}
 			}
 
@@ -53,6 +53,15 @@ public override void Check(LinkedTestCaseResult linkResult)
 			Assert.AreEqual(_assertionCounter.AssertionsMade, expectedNumberOfAssertionsToMake, $"Expected to make {expectedNumberOfAssertionsToMake} assertions, but only made {_assertionCounter.AssertionsMade}.  The test may be invalid or there may be a bug in the checking logic");
 		}
 
+		private static AssemblyContainer ReadAssembly(NPath assemblyPath)
+		{
+			var readerParams = new ReaderParameters();
+			var resolver = new AssemblyResolver();
+			readerParams.AssemblyResolver = resolver;
+			resolver.AddSearchDirectory(assemblyPath.Parent.ToString());
+			return new AssemblyContainer(AssemblyDefinition.ReadAssembly(assemblyPath.ToString(), readerParams), resolver);
+		}
+
 		private int PerformOutputAssemblyChecks (AssemblyDefinition original, NPath outputDirectory)
 		{
 			var assembliesToCheck = original.MainModule.Types.SelectMany(t => t.CustomAttributes).Where(attr => attr.IsAssemblyAssertion()).ToArray();
@@ -253,6 +262,24 @@ public override string ToString()
 			}
 		}
 
+		private class AssemblyContainer : IDisposable
+		{
+			public readonly AssemblyResolver Resolver;
+			public readonly AssemblyDefinition Definition;
+
+			public AssemblyContainer(AssemblyDefinition definition, AssemblyResolver resolver)
+			{
+				Definition = definition;
+				Resolver = resolver;
+			}
+
+			public void Dispose()
+			{
+				Resolver?.Dispose();
+				Definition?.Dispose();
+			}
+		}
+
 		private class AssertionCounter
 		{
 			public int AssertionsMade { get; private set; }

linker/Tests/Mono.Linker.Tests.Core/Mono.Linker.Tests.Core.csproj

@@ -68,6 +68,10 @@
       <Project>{d68133bd-1e63-496e-9ede-4fbdbf77b486}</Project>
       <Name>Mono.Cecil</Name>
     </ProjectReference>
+    <ProjectReference Include="..\..\Mono.Linker.csproj">
+      <Project>{dd28e2b1-057b-4b4d-a04d-b2ebd9e76e46}</Project>
+      <Name>Mono.Linker</Name>
+    </ProjectReference>
     <ProjectReference Include="..\Mono.Linker.Tests.Cases.Expectations\Mono.Linker.Tests.Cases.Expectations.csproj">
       <Project>{2c26601f-3e2f-45b9-a02f-58ee9296e19e}</Project>
       <Name>Mono.Linker.Tests.Cases.Expectations</Name>