Tests for longdouble behavior have started failing

[dfm]

In #436 we saw that the following test has started to fail:

emcee/src/emcee/tests/integration/test_longdouble.py

Lines 21 to 50 in 37dc02e

	def test_longdouble_actually_needed(cls):
	if issubclass(cls, TempHDFBackend) and not does_hdf5_support_longdouble():
	pytest.xfail("HDF5 does not support long double on this platform")
	mjd = np.longdouble(58000.0)
	sigma = 100 * np.finfo(np.longdouble).eps * mjd
	def log_prob(x):
	assert x.dtype == np.longdouble
	return -0.5 * np.sum(((x - mjd) / sigma) ** 2)
	ndim, nwalkers = 1, 20
	steps = 1000
	p0 = sigma * np.random.randn(nwalkers, ndim).astype(np.longdouble) + mjd
	assert not all(p0 == mjd)
	with cls(dtype=np.longdouble) as backend:
	sampler = emcee.EnsembleSampler(
	nwalkers, ndim, log_prob, backend=backend
	)
	sampler.run_mcmc(p0, steps)
	samples = sampler.get_chain().reshape((-1,))
	assert samples.dtype == np.longdouble
	assert not np.all(samples == mjd)
	assert np.abs(np.mean(samples) - mjd) < 10 * sigma / np.sqrt(
	len(samples)
	)
	assert 0.1 * sigma < np.std(samples) < 10 * sigma

@hendriko373 noted that the test is not seeded (which we probably should fix), but this never failed previously so I worry that something else is problematic.

@aarchiba: Since you developed this feature, do you have any thoughts about this? Thanks!