Add durations flag to coverage run on CI.

[rossbar]

This is the longest-running job so it'd be good to have a bit
more info on the test runtimes in github actions.

[rossbar]

Here's the result (copied over from the summary from the coverage job details):

============================= slowest 20 durations =============================
185.02s call     networkx/algorithms/flow/tests/test_gomory_hu.py::TestGomoryHuTree::test_les_miserables_graph_cutset
166.93s call     networkx/algorithms/approximation/tests/test_traveling_salesman.py::test_asadpour_integral_held_karp
98.31s call     networkx/algorithms/connectivity/tests/test_kcomponents.py::test_torrents_and_ferraro_graph
91.51s call     networkx/algorithms/tree/tests/test_mst.py::test_random_spanning_tree_additive_large
43.63s call     networkx/algorithms/tree/tests/test_mst.py::test_random_spanning_tree_multiplicative_large
36.83s call     networkx/algorithms/connectivity/tests/test_kcomponents.py::test_shell
26.17s call     networkx/algorithms/isomorphism/tests/test_tree_isomorphism.py::test_positive
25.48s call     networkx/algorithms/flow/tests/test_maxflow_large_graph.py::TestMaxflowLargeGraph::test_gw1
24.31s call     networkx/algorithms/connectivity/tests/test_kcomponents.py::test_random_gnp
21.12s call     networkx/tests/test_all_random_functions.py::test_rng_interface
21.03s call     networkx/algorithms/tests/test_smallworld.py::test_omega
16.24s call     networkx/algorithms/connectivity/tests/test_cuts.py::test_node_cutset_random_graphs
15.61s call     networkx/algorithms/approximation/tests/test_traveling_salesman.py::test_ascent_method_asymmetric
14.58s call     networkx/algorithms/flow/tests/test_gomory_hu.py::TestGomoryHuTree::test_davis_southern_women_graph
14.43s call     networkx/algorithms/isomorphism/tests/test_tree_isomorphism.py::test_negative
13.95s call     networkx/algorithms/flow/tests/test_gomory_hu.py::TestGomoryHuTree::test_karate_club_graph
11.05s call     networkx/algorithms/approximation/tests/test_kcomponents.py::test_example_1_detail_3_and_4
11.01s call     networkx/algorithms/tree/tests/test_branchings.py::test_arborescence_iterator_min
10.81s call     networkx/algorithms/tree/tests/test_branchings.py::test_arborescence_iterator_max
10.68s call     networkx/algorithms/approximation/tests/test_traveling_salesman.py::test_held_karp_ascent

[dschult]

It looks like GomoryHuTree accounts for many of the long tests. And the example graphs used in the testing are not strategically chosen to test aspects of that functionality. So there could be some low hanging fruit to revisit that test module.
Yay!

[dschult]

Looks like I forgot to approve this PR... so I'll do that here,