Explicitly disable a transition by setting it to false (fix #9320)

[Itisfilipe]

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 for v2.x (or to a previous version branch), not the master 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/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:
With this change will be possible to set appear to false and disable it dynamically. fix #9320

Appear set to false

Appear defined but not set

Appear not defined

Notice that consider appear to be False when absent would require either a change for the corner case or a breaking change

[Itisfilipe]

@posva no worries, feel free to completely disregard this PR. I have one question though, test-cover is working local but failing on circleci, is that something I should expect to happen in this codebase?

[posva]

Not really, the test failing is called should handle random shuffle. I haven't check the test but it could be indeed failing because of a random thing