Double check bokeh savefig test #289
Description
tests/test_plot_collection.py has a test_save_figures function in it to check the savefig method of PlotCollection. The test does pass, but it also emits this warning:
WARNING:bokeh.core.validation.check:W-1000 (MISSING_RENDERERS): Plot has no renderers: figure(id='p3047', ...)
We check the file is generated and the file is not empty but nothing else. I am worried savefig doesn't work properly with bokeh (e.g. generates non-empty but jubled html file) and we aren't catching that other than with that somewhat cryptic error.