build(deps): bump ReactiveMarbles.SourceGenerator.TestNuGetHelper

Bumps [ReactiveMarbles.SourceGenerator.TestNuGetHelper](https://github.com/reactivemarbles/SourceGenerator.TestNuGetHelper) from 1.1.1 to 1.2.1.
- [Release notes](https://github.com/reactivemarbles/SourceGenerator.TestNuGetHelper/releases)
- [Commits](reactivemarbles/SourceGenerator.TestNuGetHelper@1.1.1...1.2.1)

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

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

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

@@ -13,7 +13,7 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.4.3" PrivateAssets="all" />
     <PackageReference Include="coverlet.collector" Version="3.1.0" PrivateAssets="all" />
     <PackageReference Include="Verify.SourceGenerators" Version="1.3.0" />
-    <PackageReference Include="ReactiveMarbles.SourceGenerator.TestNuGetHelper" Version="1.1.1" />
+    <PackageReference Include="ReactiveMarbles.SourceGenerator.TestNuGetHelper" Version="1.2.1" />
     <PackageReference Include="Verify.Xunit" Version="13.1.0" />
     <PackageRefernece Include="Microsoft.CodeAnalysis.CSharp" Version="3.10.0" PrivateAssets="all" />
   </ItemGroup>