moving all the test folders back in to tests

[shimwell]

Currently the tests are split across a few folders

tests - these always run
tests_examples - these require jupyter cadquery to be installed and don't run during the conda package testing
tests_h5m - these don't run on windows as it needs pymoab
tests_show - these require jupyter cadquery to be installed and don't run during the conda package testing

However we can use pytest.mark.skipif to selectively run these. Here is an example

import sys
@pytest.mark.skipif('pymoab' not in sys.modules,
                    reason="requires the PyMoab library")
def test_h5m_function():
....

We can therefore move all the tests back to the tests folder if these decorators are added