Reassign structlit test todos, add tests

[mlevesquedion]

Consider this piece of SSA graph:

The corresponding code is:

func TestStructHoldingSourceAndInnocIsTainted(s core.Source, i core.Innocuous) {
	h := Holder{
		s,
		i,
	}
	core.Sink(h) // TODO want "a source has reached a sink"
}

Currently, the assigned issue is #97. While I think fixing #97 may solve this case, when I originally wrote these tests the intent was to capture the fact that if you put a source value into a locally allocated struct, that struct should be tainted. Now that I'm taking a fresh look at this, I believe the test case I wanted to write was:

type InterfaceHolder struct {
	i interface{}
}

func TestStructLiteralContainingTaintedInterfaceIsTainted(s core.Source) {
	ih := InterfaceHolder{
		s,
	}
	core.Sink(ih) // TODO(#212) want "a source has reached a sink"
}

The SSA graph for this one is basically the same, with an extra MakeInterface step (and without the path involving the core.Innocuous value).

In all of these cases (but this is especially clear in the cases involving interface{} fields), propagation doesn't make it through the Alloc, so taint doesn't reach the sink. To me, this seems like it's a closer fit for #212.

• (N/A) [ ] Running against a large codebase such as Kubernetes does not error out. (See DEVELOPING.md for instructions on how to do that.)
• (N/A) [ ] Appropriate changes to README are included in PR