Add picture to the isHTMLTag map so it suppresses unneeded warnings while unit testing

[timbenniks]

This change will make sure the below error is not seen while unit testing components using picture in their template: [Vue warn]: Unknown custom element: picture - did you register the component correctly? For recursive components, make sure to provide the "name" option.

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: suppresses unneeded warning while unit testing

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)