Add snowplow spec helper

What does this MR do?

This is inspired by the discussion in #238552 (closed) and the draft MR !40646 (closed) to improve snowplow testing.

The reasoning:

1. An severity1 MR to fix a bug related to Snowplow because of mismatched types !39725 (merged)
2. An severity4 MR to fix a bug related to Snowplow because of mismatched types !40628 (merged)

Both MRs could have been prevented if we actually call Snowplow instead of just mocking Gitlab::Tracking.

This is the first iteration to improve the specs around snowplow. As a follow-up iteration, we could also think about not mocking Gitlab::Tracking anymore but actually checking the requests from webmock.

Screenshots

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
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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