Inconsistent creation of blocks by test loader compared to real loader

Start in tests/testing_utils.py at the test loader implementation around lines 86-98, then inspect httomo/runner/task_runner.py around lines 158-161 and the failing test in tests/runner/test_task_runner.py. Update the loader so each produced block owns copied array data, and run the affected task-runner tests to confirm multiple blocks no longer fail after block data is deleted.

The real loader creates a new numpy array (into which a subset of the input data in the hdf5 file is copied): https://github.com/DiamondLightSource/httomo/blob/4e4b9d9bdf6e69d97aff840554ec9630b048e2c4/httomo/loaders/standard_tomo_loader.py#L171

which then eventually gets put into a DataSetBlock: https://github.com/DiamondLightSource/httomo/blob/4e4b9d9bdf6e69d97aff840554ec9630b048e2c4/httomo/loaders/standard_tomo_loader.py#L304-L313

The test loader defined in testing_utils.py which can be configured to produce one or more mock blocks:

• takes in a DataSetBlock
• and then uses slices of the numpy array within that DataSetBlock

to create new blocks: https://github.com/DiamondLightSource/httomo/blob/4e4b9d9bdf6e69d97aff840554ec9630b048e2c4/tests/testing_utils.py#L86-L98

Ie, if running httomo within a test that wants multiple blocks to be read from the test loader, the loader is providing multiple blocks containing references to a single numpy array. But if running httomo not in a test, the real loader provides multiple blocks where each block has its own data, rather than referencing a single central numpy array.

This can cause different behaviour between tests and real runs if more than one block is in a section due to the del block.data line in the task runner: https://github.com/DiamondLightSource/httomo/blob/4e4b9d9bdf6e69d97aff840554ec9630b048e2c4/httomo/runner/task_runner.py#L158-L161

For the case of multiple blocks in a test produced by the test loader, the reference count of the single central numpy array drops to 0 due to the del block.data, and subsequent attempts to read blocks fails due to the single central numpy array not being there anymore:

  File "/httomo/tests/runner/test_task_runner.py", line 735, in test_minimum_block_length_passed_to_intermediate_data_wrapper
    t._execute_section(s[0])
  File "/httomo/httomo/runner/task_runner.py", line 160, in _execute_section
    del block.data
        ^^^^^^^^^^
  File "/opt/conda/lib/python3.12/unittest/mock.py", line 823, in __delattr__
    raise AttributeError(name)
AttributeError: data

The test loader should be modified to:

• take a plain numpy array rather than a DataSetBlock
• copy data from that numpy array into a new numpy, before putting the new numpy array into a new DataSetBlock

in order to be consistent with how a real loader works when multiple blocks are loaded from it.