Try actually running the tests under .net core

LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj

@@ -1,15 +1,15 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <Import Project="$(MSBuildThisFileDirectory)..\LibZipSharp.props" />
   <PropertyGroup>
-    <TargetFrameworks>net471</TargetFrameworks>
-    <TargetFrameworks Condition="$([MSBuild]::IsOsPlatform('Windows')) And $([MSBuild]::IsOsPlatform('Linux'))">$(TargetFrameworks);netcoreapp3.1</TargetFrameworks>
+    <TargetFrameworks>net471;netcoreapp3.1</TargetFrameworks>
 
     <IsPackable>false</IsPackable>
     <LibZipSharpBundleAllNativeLibraries>true</LibZipSharpBundleAllNativeLibraries>
     <ReferenceNuget Condition="'$(ReferenceNuget)' == ''">False</ReferenceNuget>
     <DefineConstants Condition="'$(OS)' == 'Windows_NT'">$(DefineConstants);WINDOWS</DefineConstants>
   </PropertyGroup>
 
+  <Import Project="$(MSBuildThisFileDirectory)..\LibZipSharp.props" />
+
   <ItemGroup>
     <PackageReference Include="nunit" Version="3.12.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="3.16.1" />

azure-pipelines.yml

@@ -94,7 +94,7 @@ stages:
         - buildlinux
         - buildWindows
       pool:
-        vmImage: macOS-10.14
+        vmImage: macOS-latest
       steps:
         - bash: |
             brew tap xamarin/xamarin-android-windeps
@@ -194,6 +194,12 @@ stages:
           inputs:
             artifactName: NuGet
             downloadPath: $(Build.SourcesDirectory)
+        - task: MSBuild@1
+          displayName: 'Build solution LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
+          inputs:
+            solution: LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj
+            configuration: Release
+            msbuildArguments: /restore /p:ReferenceNuget=True /v:diag
         - task: MSBuild@1
           displayName: 'Build solution LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
           inputs:
@@ -204,12 +210,11 @@ stages:
           displayName: "Run Tests under .net Core"
           inputs:
             command: test
-            projects: 'LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
-            arguments: '--configuration Release --framework netcoreapp3.1 --no-build'
+            arguments: 'LibZipSharp.UnitTest/bin/Release/netcoreapp3.1/LibZipSharp.UnitTest.dll'
     - job: testmacos
       displayName: 'Test MacOS'
       pool:
-        vmImage: macOS-10.14
+        vmImage: macOS-latest
       steps:
         - template: yaml-templates/use-dot-net.yaml
           parameters:
@@ -219,16 +224,27 @@ stages:
           inputs:
             artifactName: NuGet
             downloadPath: $(Build.SourcesDirectory)
+        - task: MSBuild@1
+          displayName: 'Build solution LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
+          inputs:
+            solution: LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj
+            configuration: Release
+            msbuildArguments: /restore /p:ReferenceNuget=True /v:diag
         - task: MSBuild@1
           displayName: 'Build solution LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
           inputs:
             solution: LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj
             configuration: Release
             msbuildArguments: /restore /t:RunNunitTests /p:ReferenceNuget=True /v:diag
+        - task: DotNetCoreCLI@2
+          displayName: "Run Tests under .net Core"
+          inputs:
+            command: test
+            arguments: 'LibZipSharp.UnitTest/bin/Release/netcoreapp3.1/LibZipSharp.UnitTest.dll'
     - job: testwindows
       displayName: 'Test Windows'
       pool:
-        vmImage: windows-2019
+        vmImage: windows-latest
       steps:
         - template: yaml-templates/use-dot-net.yaml
           parameters:
@@ -240,6 +256,12 @@ stages:
             downloadPath: $(Build.SourcesDirectory)
         - task: MSBuild@1
           displayName: 'Build solution LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
+          inputs:
+            solution: LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj
+            configuration: Release
+            msbuildArguments: /restore /p:ReferenceNuget=True /v:diag
+        - task: MSBuild@1
+          displayName: 'Run Tests LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
           inputs:
             solution: LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj
             configuration: Release
@@ -248,8 +270,8 @@ stages:
           displayName: "Run Tests under .net Core"
           inputs:
             command: test
-            projects: 'LibZipSharp.UnitTest/LibZipSharp.UnitTest.csproj'
-            arguments: '--configuration Release --framework netcoreapp3.1 --no-build'
+            arguments: 'LibZipSharp.UnitTest\bin\Release\netcoreapp3.1\LibZipSharp.UnitTest.dll
+'
   - stage: Publish
     dependsOn: Test
     condition: eq(variables['System.TeamProject'], 'devdiv') # only sign the packages when running on Windows, and using the private server which has the certificates