Add test suite for useDefinition hook in @backstage/ui (#34042)

* Add test infrastructure to @backstage/ui

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add resolveResponsiveValue tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition prop resolution and classes tests

Fixed a bug in useDefinition where passing null for classNameTarget or
utilityTarget was incorrectly defaulted to 'root' due to the nullish
coalescing operator treating null as falsy.

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition data attributes tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add resolveDefinitionProps tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add processUtilityProps tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition bg system tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition utility style and analytics tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition href resolution tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Add useDefinition options tests

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Wrap all useDefinition tests with BUIProvider

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Fix type errors in useDefinition tests

Use `as const satisfies ComponentConfig<any, any>` instead of
`as ComponentConfig<any, any>` to preserve literal types needed
by the conditional type machinery (DataAttributes, ResolveBgProps,
analytics intersection).

Signed-off-by: Johan Persson <johanopersson@gmail.com>

* Address review feedback in useDefinition tests

- Rename shadowed Wrapper to RouterWrapper in createRouterWrapper
- Fix inaccurate comment about splat vs non-splat routes
- Clarify misleading test name for provider data-bg behavior

Signed-off-by: Johan Persson <johanopersson@gmail.com>

---------

Signed-off-by: Johan Persson <johanopersson@gmail.com>

packages/ui/package.json

@@ -59,6 +59,8 @@
   "devDependencies": {
     "@backstage/cli": "workspace:^",
     "@storybook/react-vite": "^10.3.3",
+    "@testing-library/jest-dom": "^6.0.0",
+    "@testing-library/react": "^16.0.0",
     "@types/react": "^18.0.0",
     "@types/react-dom": "^18.0.0",
     "@types/use-sync-external-store": "^1.0.0",