Visualization test crashes if dot not installed

[heplesser]

The network visualization tests crashes if dot is not available. The test has a condition on pydot being importable

nest-simulator/testsuite/pytests/test_visualization.py

Lines 42 to 48 in c201d67

	import pydot # noqa: F401
	HAVE_PYDOT = True
	except ImportError:
	HAVE_PYDOT = False
	try:

but this does not check if there actually is a runable dot available. To do this, one would need to add a try containing

pydot.Dot().write_png('foo.png')

catching FileNotFoundError and a cleanup removing foo.png if the writing succeeded.

[med-ayssar]

shutil.which("exe_name") is None would be a much easier solution to find whether an executable is available under PATH or not.