fix: correctly find functional components

[lmiller1990]

Attempting to fix #1820, grabbed some work from https://github.com/vuejs/vue-test-utils/pull/1832/files and some additional code from the original issue. This seems to satisfy all the test cases.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[Bastczuak]

@lmiller1990 looks superb dude! but why is this specific test, I also had my problems with, failing in test:compat but not in test:unit:browser? Where is the difference?

[lmiller1990]

@Bastczuak test-compat will run the suite against all the older versions of Vue, lik v2.1, v2.2, v2.3... I have no idea why that router tests fails on versions v2.5 and older, I guess some internals changed. The matching logic goes super deep into various undocumented internals.

v2.6 is over two years old, we can probably afford to drop support for some of those edge cases. I'd like for some people to run this against their production code bases to see if I've broken anything else, I am not working full-time with Vue at the moment and don't have any huge production tes suites to run this against.