Fix some minor annoyances in Python tests

[ricrogz]

I usually work on a Qt-based out-of-tree installation, and this patch addresses some issues in Python tests I have been ignoring for a (long) while:

• The github4823 test is configured to run in-tree, and fails due to a rdkit import error when using an out-of-tree installation with RDK_INSTALL_INTREE disabled and RDK_INSTALL_PYTHON_TESTS enabled.

- UnitTestDraw.py::_testMolToFile() and UnitTestDraw.py::_testMolToImage() were not being run due to the test names being prefixed with an underscore. I renamed them and skipped them when Cairo is not available, since they both require MolToImage() at some point. MolToImage() is not available on Qt builds due to Cairo/Qt incompatibility. Whoops, these aren't meant to be tests, they are functions used by other tests...

• PandasTools.py::testGithub1507() (both instances): As explained by the comment, SaveXlsxFromFrame() uses MolToImage() to generate PNG images that are embedded into the Xlsx file, and due to the incompatibility mentioned above, it won't work in Qt-based installations.

• UnitTestPandasTools.py::test_github2380(): when run under ctest -R pythonSourceTests, this test fails in a Qt installation due to a test-test interaction with Chem/Draw/UnitTestIPython.py: when running pythonSourceTests,
  Chem/Draw/UnitTestIPython.py runs before UnitTestPandasTools.py::test_github2380(). It imports IPythonConsole, which sets up the IPython renderer (which allows avoiding the Qt/Cairo incompatibility by doing Draw.MolsToGridImage = IPythonConsole.ShowMols and drawing SVGs instead of PNGs), and then uninstalls it as the test ends. Since tests are all run in the same process, when UnitTestPandasTools.py::test_github2380() runs, the from rdkit.Chem.Draw import IPythonConsole call does not set up the renderer because the IPythonConsole module is still loaded, so the init won't trigger again, but since the rendered was uninstalled, Draw.MolsToGridImage() has been restored to the default implementation, which uses MolToImage(), triggering the failure. My fix ensures that the IPython rendered is installed before PandasTools.FrameToGridImage() is called, and then cleans it up after it runs.

[greglandrum]

LGTM