Bump NUnit from 3.13.3 to 4.0.1

Bumps [NUnit](https://github.com/nunit/nunit) from 3.13.3 to 4.0.1.
- [Release notes](https://github.com/nunit/nunit/releases)
- [Changelog](https://github.com/nunit/nunit/blob/master/CHANGES.md)
- [Commits](nunit/nunit@v3.13.3...v4.0.1)

---
updated-dependencies:
- dependency-name: NUnit
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

src/Symbolics.Tests/Symbolics.Tests.fsproj

@@ -30,7 +30,7 @@
     <Compile Include="Main.fs" />
   </ItemGroup>
   <ItemGroup>
-    <PackageReference Include="NUnit" Version="3.13.3" />
+    <PackageReference Include="NUnit" Version="4.0.1" />
     <PackageReference Include="NUnit3TestAdapter" Version="4.4.2" />
     <PackageReference Include="NUnitLite" Version="3.13.3" />
     <PackageReference Include="FsUnit" Version="5.3.0" />