Bump the tests group with 1 update

Bumps Microsoft.NET.Test.Sdk from 18.3.0 to 18.4.0

---
updated-dependencies:
- dependency-name: Microsoft.NET.Test.Sdk
  dependency-version: 18.4.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: tests
...

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

Author: dependabot[bot]

src/Schematron.Tests/Schematron.Tests.csproj

@@ -11,7 +11,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.3.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="18.4.0" />
     <PackageReference Include="xunit" Version="2.*" />
     <PackageReference Include="xunit.runner.visualstudio" Version="3.1.5" />
   </ItemGroup>