Enable dynamic PHPUnit downloading of given version

Directory.Build.props

@@ -1,7 +1,7 @@
 <Project>
 
   <PropertyGroup>
-    <PhpUnitVersion>9.2.3</PhpUnitVersion>
+    <PhpUnitVersion Condition="$(PhpUnitVersion) == ''">9.2.3</PhpUnitVersion>
   </PropertyGroup>
 
 </Project>

src/Peachpied.PhpUnit/Peachpied.PhpUnit.msbuildproj

@@ -6,8 +6,16 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <Compile Include="*.phar" />
     <None Include="LICENSE.txt" Pack="true" PackagePath=""/>
   </ItemGroup>
 
+  <Target Name="DownloadPhpUnit" BeforeTargets="CoreCompile">
+    <DownloadFile
+      SourceUrl="https://phar.phpunit.de/phpunit-$(PhpUnitVersion).phar"
+      DestinationFolder="$(MSBuildThisFileDirectory)"
+      DestinationFileName="phpunit.phar">
+      <Output TaskParameter="DownloadedFile" ItemName="Compile" />
+    </DownloadFile>
+  </Target>
+
 </Project>