build(deps): bump ReactiveMarbles.SourceGenerator.TestNuGetHelper (#144)

Bumps [ReactiveMarbles.SourceGenerator.TestNuGetHelper](https://github.com/reactivemarbles/SourceGenerator.TestNuGetHelper) from 1.2.1 to 1.2.21.
- [Release notes](https://github.com/reactivemarbles/SourceGenerator.TestNuGetHelper/releases)
- [Commits](reactivemarbles/SourceGenerator.TestNuGetHelper@1.2.1...1.2.21)

---
updated-dependencies:
- dependency-name: ReactiveMarbles.SourceGenerator.TestNuGetHelper
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

src/Splat.DependencyInjection.SourceGenerator.Tests/Splat.DependencyInjection.SourceGenerator.Tests.csproj

@@ -13,7 +13,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.5" PrivateAssets="all" />
     <PackageReference Include="coverlet.msbuild" Version="3.1.2" />
     <PackageReference Include="Verify.SourceGenerators" Version="1.5.0" />
-    <PackageReference Include="ReactiveMarbles.SourceGenerator.TestNuGetHelper" Version="1.2.1" />
+    <PackageReference Include="ReactiveMarbles.SourceGenerator.TestNuGetHelper" Version="1.2.21" />
     <PackageReference Include="Verify.Xunit" Version="17.1.6" />
     <PackageRefernece Include="Microsoft.CodeAnalysis.CSharp" Version="3.10.0" PrivateAssets="all" />
   </ItemGroup>