math/rand: tests are too weak due to misuse of statsResults.maxError

The math/rand tests use the function nearEqual to compare floats:

func nearEqual(a, b, closeEnough, maxError float64) bool {
	absDiff := math.Abs(a - b)
	if absDiff < closeEnough { // Necessary when one value is zero and one value is close to zero.
		return true
	}
	return absDiff/max(math.Abs(a), math.Abs(b)) < maxError
}

A bit of thought suggests that maxError should always be pretty small, if this function is ever to return false. However, maxError is usually scaled everywhere it is used (e.g. stddev * 0.08), and in practice it frequently ends up >> 2.

As a result, many of the math/rand tests have no teeth. An extreme example of this is TestReadUniformity, which requires (among other things) that the mean of just 2 randomly selected bytes must be very near 255.0/2, a test which should almost always fail, but which currently succeeds, due to a large value of maxError. (Related: CL 51310.)

Replacing maxError with a constant 0.08 causes most math/rand tests to pass, except TestReadUniformity when called with small n (in which case it should fail).

However, I don't have enough domain expertise to know whether this is reasonable or whether an alternative bound would be better. Advice requested.

golang.org/s/owners lists no one for math/rand, so I'll guess and cc: @rsc @aclements @robpike