This improves the pytest test discovery with the following points,

• Running pytest with no arguments (or pytest .) now correctly collects all available tests (except for emsdk ones which require the compiler setup and have to be run separately with make -C emsdk test)
• Running pytest on packages now skips package tests that are not built. This is done at the selenium fixture level and so will also apply to new tests.
• packages/test_packages_common.py::test_import now will skip the test before loading the selenium_standalone fixture which is slow. Previously it was first loading the slow fixture, and then skipping.
• removes make test as I think pytest should not be used via make, as then one loses access to all useful flags (e.g. --pdb, -n auto (for parallelism), -s for printing stdout, etc)

Closes #1979