Add test case from #117 checking for varargs ambiguity

[PhilipVinc]

I recently noticed that for some reason a test case I had added in #119 that previously did not pass test, now pass.

The test is that the following raises an AmbiguousLookupError

def f(x : int, *y: A):
 pass
def f(x: int, *y : B):
 pass

f(1)

I would like to add this to the test set of plum just so that it's included.

Note that #117 is still not solved as we still do not pass this other test

assert Sig(int, varargs=int) < Sig(int, Num)

but that's a step in the right direction...

[coveralls]

Pull Request Test Coverage Report for Build 9077591603

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.919%

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Totals
Change from base Build 9075861722:	0.0%
Covered Lines:	1226
Relevant Lines:	1227

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💛 - Coveralls

[wesselb]

Thanks, this looks great! :) Merging right away.