Use high-quality icon for vsix

TestGeneration.Extensions.NUnit/TestGeneration.Extensions.NUnit.csproj

@@ -62,7 +62,7 @@
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
       <IncludeInVSIX>true</IncludeInVSIX>
     </Content>
-    <Content Include="nunit3_32x32.png">
+    <Content Include="nunit_90.png">
       <CopyToOutputDirectory>Always</CopyToOutputDirectory>
       <IncludeInVSIX>true</IncludeInVSIX>
     </Content>

TestGeneration.Extensions.NUnit/source.extension.vsixmanifest

@@ -10,7 +10,7 @@ Works on Visual Studio from version 2015.
     <MoreInfo>https://github.com/nunit/nunit-vs-testgenerator</MoreInfo>
     <License>license.txt</License>
     <GettingStartedGuide>https://github.com/nunit/docs/wiki/Visual-Studio-Test-Generator</GettingStartedGuide>
-    <Icon>nunit3_32x32.png</Icon>
+    <Icon>nunit_90.png</Icon>
     <PreviewImage>preview.jpg</PreviewImage>
     <Tags>unit testing, NUnit</Tags>
   </Metadata>