tests: improve output of "pytest -v"

[woutdenolf]

pytest -v

currently gives

dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file0] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file1] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file2] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file3] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file4] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file5] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file6] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file7] PASSED                                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[nxdl_file8] PASSED                                        [  0%]
...

and after this PR

dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXapm.nxdl.xml] PASSED                       [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXarchive.nxdl.xml] PASSED                   [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXarpes.nxdl.xml] PASSED                     [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXazint1d.nxdl.xml] PASSED                   [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXazint2d.nxdl.xml] PASSED                   [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXcanSAS.nxdl.xml] PASSED                    [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXdirecttof.nxdl.xml] PASSED                 [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXellipsometry.nxdl.xml] PASSED              [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXem.nxdl.xml] PASSED                        [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXfluo.nxdl.xml] PASSED                      [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXindirecttof.nxdl.xml] PASSED               [  0%]
dev_tools/tests/test_docs.py::test_nxdl_generate_doc[applications/NXiqproc.nxdl.xml] PASSED                    [  0%]