[Security Solution] Rework test plan for importing prebuilt rules

[banderror]

Epic: #174168
Partially addresses: #202079, #210358

Summary

We started to rework and introduce functional changes to our existing test plans for prebuilt rule customization, upgrade, and export/import workflows.

Specifically, this PR:

• Restructures the test plan, introduces a more fine-grained list of sections.
• Rewrites almost all the existing scenarios. In most cases it boils down to splitting a scenario into 2+ more specific scenarios, where each describes exactly what happens in the GIVEN and THEN sections. This is very important, as it:
  • makes these scenarios ready to be implemented right away: 1 scenario = 1 test to write
  • helps with ensuring that we covered edge cases
• Adds new scenarios for handling missing base versions according to #210358.
• Adds a placeholder section for licensing scenarios according to #11502.
• Removes scenarios for the overwrite request parameter - this is common importing logic which is not related to prebuilt rules.
• Addresses my own comments from this review.

The new test plan should be in line with the changes discussed in #210358.

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[elasticmachine]

Pinging @elastic/security-detection-rule-management (Team:Detection Rule Management)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 87353e9

cc @banderror