[ci][tests][python] remove assertion for filename that is no longer true with new version of graphviz

[StrikerRUS]

New 0.18 version of graphviz was released about 7 hours ago: https://pypi.org/project/graphviz/#history.
In new version something was broken (I guess due to refactoring the internal class hierarchy) and default value for the filename argument was changed.

filename – Filename for saving the source (defaults to name + '.gv').
https://graphviz.readthedocs.io/en/stable/api.html#graphviz.Digraph

So our test started to fail with the following assertion error:

        graph.render(view=False)
        assert isinstance(graph, graphviz.Digraph)
        assert graph.name == 'Tree4'
>       assert graph.filename == 'Tree4.gv'
E       AssertionError: assert 'Digraph.gv' == 'Tree4.gv'
E         - Tree4.gv
E         + Digraph.gv

https://github.com/microsoft/LightGBM/runs/4131382043?check_suite_focus=true#step:5:1686

I suggest simply remove this assertion as it doesn't related to LightGBM API as we just pass kwargs to Digraph constructor as-is:

LightGBM/python-package/lightgbm/plotting.py

Lines 664 to 666 in aafedd8

	**kwargs
	Other parameters passed to ``Digraph`` constructor.
	Check https://graphviz.readthedocs.io/en/stable/api.html#digraph for the full list of supported parameters.

... <some calls of internal functions that don't modify kwargs in any way >

LightGBM/python-package/lightgbm/plotting.py

Line 496 in aafedd8

graph = Digraph(**kwargs)

[jameslamb]

agree! Thanks for investigating this.

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