Split KGML graphics tests into online and offline modules

Following on from [PR #451](https://github.com/biopython/biopython/pull/451), split KGML graphics tests into online and offline modules.

Tests pass on my machine:

```
$ python test_KGML_graphics_online.py
test_render_KGML_import_map (__main__.KGMLPathwayOnlineTest)
Basic rendering of KGML: use imported imagemap ... ok

----------------------------------------------------------------------
Ran 1 test in 27.549s

OK
```