[Code health] Packing Structure of Unit Test

Currently, some of the files are there at the parent level of the unit test module, which can be cluttered if increases in the future (As the code grows, the files definitely increase in this area)

This can be fixed by putting each file in their respective packages. This re-structuring will not affect what packaging structure we are following in the main module, as all these are helper classes for testing purpose only.

Suggestion
```md
unitTest/java/com/google/android/ground
├── domain
├── hilt
│ ├── BaseHiltTest
│ └── HiltExt.kt
├── mockito
│ └── MockitoKotlinHelpers.kt
├── model
├── module
│ ├── TestAuthenticationModule
│ ├── TestCoroutineDispatchersModule
│ ├── TestCoroutineScopesModule
│ ├── TestRemoteStorageModule
│ ├── TestWorkManagerModule
│ └── TestLocalDatabaseModule
├── navigation
│ └── NavigationTestHelper.kt
├── persistence
├── repository
├── system
├── ui
├── util
│ └── ResultAssertions.kt
├── view
│ ├── RecyclerViewHelper.kt
│ ├── CustomViewActions
│ └── UiTestHelper.kt
├── MainActivityTest
└── MainViewModelTest
```