wifi-he-network tests could be moved to EXTENSIVE

We have been observing some long duration tests when running ./test.py in default unit test mode (Tommaso has also reported similar concerns with weekly tests). Valgrind tests take many days to complete on our Jenkins server as well.

For comparison, ns-3.33 unit tests (examples and tests) take about 3 minutes to run in debug mode on an Intel Xeon Gold 6126 (running ./test.py -d), while current ns-3-dev tests took 106 minutes on the same machine.

The following three tests are of particular concern; most others complete less than 6-7 minutes. Times reported below are in seconds (e.g. 3469.633 seconds).

[729/731] ^[[32mPASS^[[0m (3469.633): Example examples/wireless/wifi-he-network --simulationTime=0.25 --frequency=5 --useRts=0 --minExpectedThroughput=6 --maxExpectedThroughput=844
[730/731] ^[[32mPASS^[[0m (3616.274): Example examples/wireless/wifi-he-network --simulationTime=0.3 --frequency=5 --useRts=0 --useExtendedBlockAck=1 --minExpectedThroughput=6 --maxExpectedThroughput=1033
[731/731] ^[[32mPASS^[[0m (6352.131): Example examples/wireless/wifi-he-network --simulationTime=0.3 --frequency=5 --useRts=1 --minExpectedThroughput=6 --maxExpectedThroughput=745

These appear to be recent regressions, since the last batch of wifi commits.