Use assert ... is True in XFAIL tests

[oscarbenjamin]

Brief description of what is fixed or changed

Changes failing asserts in xfail tests to use assert ... is True rather than just assert .... Otherwise the result is assert None which triggers a warning under pytest

Other comments

I raised this as an issue with pytest: pytest-dev/pytest#4639.

Release Notes

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#### Brief description of what is fixed or changed

Changes failing asserts in xfail tests to use `assert ... is True` rather than just `assert ...`. Otherwise the result is `assert None` which triggers a warning under pytest

#### Other comments

I raised this as an issue with `pytest`: [https://github.com/pytest-dev/pytest/issues/4639](https://github.com/pytest-dev/pytest/issues/4639).

#### Release Notes

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[asmeurer]

How about

Suggested change

	raise NotImplementedError("deltaintegrate(f((x + 2)/5)*DiracDelta((x - 2)/3) - g(x)*diff(DiracDelta(x - 1), x), (x, 0, 3))")
	assert deltaintegrate(f((x + 2)/5)*DiracDelta((x - 2)/3) - g(x)*diff(DiracDelta(x - 1), x), (x, 0, 3)) is not None

(or better, assert that it equals the correct answer).

XFAIL tests should ideally xpass once they start working.

[oscarbenjamin]

What you're saying makes sense and I fully agree. I think that an xfail test should not only xpass when working but should actually be a reasonable non-xfail test of the correct behaviour.

Happy to change it but if you look at the surrounding code though you'll see why I did it like this.

[asmeurer]

I actually don't understand this test. Why is it testing the internal deltaintegrate function? For whatever reason, it doesn't work, even if you split up the integral, but integrate itself works just fine:

>>> integrate(f((x + 2)/5)*DiracDelta((x - 2)/3) - g(x)*diff(DiracDelta(x - 1), x), (x, 0, 3))
3*f(4/5) + Subs(Derivative(g(x), x), x, 1)

The Wester tests should be testing the public APIs.

So I actually think we should change this test to a passing one that tests the above.

[asmeurer]

Suggested change

	raise NotImplementedError("deltaintegrate(f((x + 2)/5)*DiracDelta((x - 2)/3) - g(x)*diff(DiracDelta(x - 1), x), (x, 0, 3))")
	assert integrate(f((x + 2)/5)*DiracDelta((x - 2)/3) - g(x)*diff(DiracDelta(x - 1), x), (x, 0, 3)) == 3*f(S(4)/5) + Subs(Derivative(g(x), x), x, 1)

[asmeurer]

I have just the one comment. I don't really understand why pytest has an issue with assert None, but this seems fine.

[oscarbenjamin]

I don't agree with pytest's issue with assert None. It seems unnecessary but at the same time I can see the argument that it is better to be explicit in tests and write assert f() is True rather than assert f() which almost any random object would pass.