Dashboard specs cleanup of mock data and axios calls

What does this MR do?

Gives ❤ to our dashboard specs. Also:

Clean up dashboard specs to match latest changes

• Renames metricsGroupsAPIResponse to metricsDashboardPayload to better represent the meaning of this piece of mock data as a vuex store payload.
• Removes several unused axios setups (mock.onGet) in order to clean make use of the vuex store as dependency instead of a mock backend.
• Remove redundant occurrences of wrapper.destroy()

I scheduled for %12.8 as this is lower priority than other items

Screenshots

N/A

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #195670 (closed)