Add a new `test.AssertNil()` helper to facilitate asserting that a given
unit test result is a non-boxed nil. Update `test.AssertNotNil()` to use
the reflect package's `.IsNil()` method to catch boxed nils.
In Go, variables whose type is constrained to be an interface type (e.g.
a function parameter which takes an interface, or the return value of a
function which returns `error`, itself an interface type) should
actually be thought of as a (T, V) tuple, where T is their underlying
concrete type and V is their underlying value. Thus, there are two ways
for such a variable to be nil-like: it can be truly nil where T=nil and
V is uninitialized, or it can be a "boxed nil" where T is a nillable
type such as a pointer or a slice and V=nil.
Unfortunately, only the former of these is == nil. The latter is the
cause of frequent bugs, programmer frustration, a whole entry in the Go
FAQ, and considerable design effort to remove from Go 2.
Therefore these two test helpers both call `t.Fatal()` when passed a
boxed nil. We want to avoid passing around boxed nils whenever possible,
and having our tests fail whenever we do is a good way to enforce good
nil hygiene.
Fixes#3279
4ad66729d2