Some UI layer tests take longer than expected to run

Start by running pytest --durations=0 tests/test_ui_layer.py and inspect the five slowest cases in tests/test_ui_layer.py, especially the pipeline-building tests. Trace what makes those cases slower than the others and verify that the full test file still passes with reduced durations and unchanged behavior.

The top 5 stick out, in comparison to the rest of tests which take 0.01s or less:

(base) root@57b6bd69e7ba:/httomo# pytest --durations=0 tests/test_ui_layer.py
========================================== test session starts ===========================================
platform linux -- Python 3.12.8, pytest-8.3.4, pluggy-1.5.0 -- /opt/conda/bin/python3.12
cachedir: .pytest_cache
rootdir: /httomo
configfile: pyproject.toml
plugins: xdist-3.6.1, mpi-0.6, mock-3.14.0, cov-6.0.0
collected 20 items

tests/test_ui_layer.py::test_can_read_cpu_pipeline PASSED                                          [  5%]
tests/test_ui_layer.py::test_can_read_gpu_pipeline PASSED                                          [ 10%]
tests/test_ui_layer.py::test_uilayer_fails_with_nonexistant_file[does_not_exist.yaml] PASSED       [ 15%]
tests/test_ui_layer.py::test_pipeline_build_no_loader PASSED                                       [ 20%]
tests/test_ui_layer.py::test_pipeline_build_duplicate_id PASSED                                    [ 25%]
tests/test_ui_layer.py::test_pipeline_build_cpu_pipeline PASSED                                    [ 30%]
tests/test_ui_layer.py::test_build_pipeline_from_json PASSED                                       [ 35%]
tests/test_ui_layer.py::test_build_pipeline_with_param_sweeps[manual] PASSED                       [ 40%]
tests/test_ui_layer.py::test_build_pipeline_with_param_sweeps[range] PASSED                        [ 45%]
tests/test_ui_layer.py::test_raise_error_if_pipeline_is_yaml_string PASSED                         [ 50%]
tests/test_ui_layer.py::test_raise_error_if_pipeline_is_json_file PASSED                           [ 55%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[no_ref] PASSED                  [ 60%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[as ${{mixed_with_other}}] PASSED [ 65%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[${{mixed_with_other}} asd] PASSED [ 70%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[${{}}] PASSED                   [ 75%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[${{with spaces}}] PASSED        [ 80%]
tests/test_ui_layer.py::test_update_side_output_references_invalid[${{inV-1%^&valid_chars}}] PASSED [ 85%]
tests/test_ui_layer.py::test_update_side_output_references_normal PASSED                           [ 90%]
tests/test_ui_layer.py::test_update_side_output_references_nosidestr PASSED                        [ 95%]
tests/test_ui_layer.py::test_update_side_output_references_notfound PASSED                         [100%]

=========================================== slowest durations ============================================
1.02s call     tests/test_ui_layer.py::test_pipeline_build_cpu_pipeline
0.79s call     tests/test_ui_layer.py::test_pipeline_build_duplicate_id
0.36s call     tests/test_ui_layer.py::test_build_pipeline_with_param_sweeps[range]
0.32s call     tests/test_ui_layer.py::test_build_pipeline_with_param_sweeps[manual]
0.23s call     tests/test_ui_layer.py::test_build_pipeline_from_json
0.02s call     tests/test_ui_layer.py::test_can_read_cpu_pipeline
0.01s setup    tests/test_ui_layer.py::test_raise_error_if_pipeline_is_json_file
0.01s call     tests/test_ui_layer.py::test_pipeline_build_no_loader
0.01s call     tests/test_ui_layer.py::test_can_read_gpu_pipeline
0.01s call     tests/test_ui_layer.py::test_update_side_output_references_notfound
0.01s call     tests/test_ui_layer.py::test_update_side_output_references_normal

(49 durations < 0.005s hidden.  Use -vv to show these durations.)
=========================================== 20 passed in 2.91s ===========================================