[#66563] Use custom ESBuild test builder

Use the custom-esbuild unit-test builder so Vitest reuses the
application esbuild plugins during test execution.

Analogous to e6b2ccaaa9.
See: https://github.com/just-jeb/angular-builders

https://community.openproject.org/wp/66563

frontend/angular.json

@@ -146,11 +146,10 @@
           }
         },
         "test": {
-          "builder": "@angular/build:unit-test",
+          "builder": "@angular-builders/custom-esbuild:unit-test",
           "options": {
             "tsConfig": "tsconfig.spec.json",
-            "buildTarget": "::development",
-            "runner": "vitest",
+            "buildTarget": "OpenProject:build",
             "providersFile": "src/test-providers.ts",
             "setupFiles": ["src/test-setup.ts"]
           }