Unexpected behavior with legacy context (both shallow and mount)

### Current behavior

Cannot test legacy context behavior (mount and shallow, on the providing and on the consuming component) → `.context` either throws or returns an empty context depending on how it's used.

### Expected behavior

`.context` returns the context somehow (after `.dive` or not, I'm not positive what the correct behavior should be)

### Your environment

Original issue was on React 16.2. Reproduction on React 16.8: https://codesandbox.io/s/oqo7nvxp2q (Notice the app works but the tests fail)

#### API

- [x] shallow
- [x] mount
- [ ] render

#### Version

| library | version
| ------------------- | -------
| enzyme | 3.9.0
| react | 16.8.6
| react-dom | 16.8.6
| react-test-renderer | N/A
| adapter (below) | 1.12.1 (react-16)

#### Adapter

- [x] enzyme-adapter-react-16
- [ ] enzyme-adapter-react-16.3
- [ ] enzyme-adapter-react-16.2 (← also the case here, but not for the above code sandbox)
- [ ] enzyme-adapter-react-16.1
- [ ] enzyme-adapter-react-15
- [ ] enzyme-adapter-react-15.4
- [ ] enzyme-adapter-react-14
- [ ] enzyme-adapter-react-13
- [ ] enzyme-adapter-react-helper
- [ ] others ( )

Start with the linked CodeSandbox reproduction and compare `.context` behavior for shallow and mount, on both providing and consuming components. Read the React 16 adapter and existing traversal tests for related context handling. Done means the expected legacy-context behavior is decided, reproduced in regression tests, and no longer throws or returns an unexpectedly empty context.

Written by the indexing model from the issue text.