Add unit tests for `TimeSnapshot` command in `GeneralDiagnosticsCluster` (#41291)

* Add unit tests for `TimeSnapshot` command in `GeneralDiagnosticsCluster`

- Introduced a mock CommandHandler and a NullProvider for testing.
- Implemented tests for TimeSnapshot command, including scenarios with and without POSIX time support.
- Verified response values and ensured correct handling of command invocations.
- Added helper methods for setting up TLV readers and buffers for command testing.

* Refactor `TestGeneralDiagnosticsCluster` to improve command invocation testing

- Moved command invocation logic into a helper method  for better code reuse and clarity.

* Handle status responses in `ClusterTester` when `InvokeCommand` returns `nullopt`

When InvokeCommand returns nullopt, check for both data responses and
status responses in the mock handler. Previously only data responses
were checked. Now if a status response exists, use the last status
from the handler.

* Handle status responses and unexpected states in ClusterTester

When InvokeCommand returns nullopt, check for both data responses
and status responses. If a status response exists, use the last
status from the handler. Also handle the unexpected case where
neither response nor status is provided by setting an error status
and logging a warning.

* Refactor `TestGeneralDiagnosticsCluster` to improve command invocation and response handling

- Moved the `InvokeTimeSnapshotAndGetResponse` method outside the test struct for better organization and clarity.
- Simplified the command invocation process by ensuring `ClusterTester` is instantiated within each test case.
- Enhanced readability by removing redundant code and ensuring consistent error handling during command execution.

* Refactor `InvokeTimeSnapshotAndGetResponse` to use `ClusterTester` directly in tests

- Updated the `InvokeTimeSnapshotAndGetResponse` function to accept `ClusterTester` instead of `GeneralDiagnosticsCluster` for improved clarity and consistency in test cases.
- Adjusted all relevant test cases to reflect this change, ensuring proper command invocation and response handling.

* Enhance error handling in `InvokeTimeSnapshotAndGetResponse` for improved robustness

- Updated the InvokeTimeSnapshotAndGetResponse function to return appropriate error codes for various failure scenarios, ensuring better error handling during command execution.
- Adjusted test cases to assert the return value of the function, enhancing the reliability of the tests and ensuring that errors are properly captured and reported.

* Fix test helper to properly propagate errors

Changed InvokeTimeSnapshotAndGetResponse() to return CHIP_ERROR instead
of void. ASSERT macros inside void helper functions only return from the
helper, not from the test itself, which can lead to tests continuing with
uninitialized data and undefined behavior.