fix($test): change to mocked names

[shigma]

Summary

VuePress uses a temporary workaround for jest cannot resolve module path from 'mocks' when using 'require.resolve'. However, it will break all the tests on external packages.

Example: https://circleci.com/gh/vuepress/vuepress-plugin-mathjax/5

In this PR, VuePress module loader will only workaround under jest and for plugins and themes that matches the following regular expression:

const MOCKED_REGEX = /^(@vuepress\/|(@[\w-]+\/)?vuepress-)(theme|plugin)-mocked-/

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

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

If changing the UI of default theme, please provide the before/after screenshot:

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:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

N/A

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

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

N/A

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[ulivz]

Closing this in favor of: 5d2fc90