Bump diffenginetray in /tests/NHapi.NUnit.SourceGeneration

Bumps [diffenginetray](https://github.com/VerifyTests/DiffEngine) from 15.8.0 to 15.9.0.
- [Release notes](https://github.com/VerifyTests/DiffEngine/releases)
- [Commits](VerifyTests/DiffEngine@15.8.0...15.9.0)

---
updated-dependencies:
- dependency-name: diffenginetray
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

.config/dotnet-tools.json

@@ -3,7 +3,7 @@
   "isRoot": true,
   "tools": {
     "diffenginetray": {
-      "version": "15.8.0",
+      "version": "15.9.0",
       "commands": [
         "DiffEngineTray"
       ]