Investigate how to speed up the test suite

[danielhollas]

I noticed that the test suite takes a significantly longer time with aiida-core versions 2.5 and 2.6. It would be good to investigate why that is and fix it, either in aiida-core if possible, or in the test suite.

This may also in general indicate that there are improvements that could be made to make the tests run faster. Here are some timings from pytest pointing to the slowest tests.

============================= slowest 30 durations =============================
70.44s call     tests/test_cli.py::test_download_and_install_pseudos
23.61s call     tests/test_pseudo.py::test_download_and_install_pseudo_from_file
20.53s call     tests/test_pseudo.py::test_pseudos_installation
15.71s setup    tests/test_app.py::test_reload_and_reset
12.31s call     tests/test_plugins_bands.py::test_result
11.03s call     tests/test_plugins_pdos.py::test_result
10.83s call     tests/test_plugins_bands.py::test_structure_1d
10.74s call     tests/test_result.py::test_summary_view
10.69s call     tests/test_plugins_bands.py::test_structure_2d
10.62s call     tests/test_result.py::test_workchainview
10.53s call     tests/test_plugins_electronic_structure.py::test_electronic_structure
10.45s call     tests/test_result.py::test_summary_report_advanced_settings
9.69s call     tests/test_result.py::test_summary_report
8.90s call     tests/test_app.py::test_reload_and_reset
8.58s call     tests/test_result.py::test_result_step
6.07s setup    tests/test_submit_qe_workchain.py::test_create_builder_insulator
5.90s setup    tests/test_plugins_bands.py::test_result
5.87s setup    tests/test_codes.py::test_code_not_selected
5.76s setup    tests/test_submit_qe_workchain.py::test_create_builder_default
5.71s setup    tests/test_codes.py::test_identify_submission_blockers
5.58s setup    tests/test_plugins_bands.py::test_structure_1d
5.58s setup    tests/test_submit_qe_workchain.py::test_create_builder_advanced_settings
5.56s setup    tests/test_codes.py::test_set_selected_codes
5.55s setup    tests/test_plugins_bands.py::test_structure_2d
5.54s setup    tests/test_plugins_pdos.py::test_result
5.53s setup    tests/test_plugins_xas.py::test_settings
4.97s setup    tests/test_plugins_xps.py::test_settings
4.49s setup    tests/test_pseudo.py::test_pseudos_setter_widget
3.97s call     tests/test_submit_qe_workchain.py::test_create_builder_default
3.46s setup    tests/test_result.py::test_result_step

It's suspicious that there seems to be a lot of slow fixtures (those lines with setup)