[Fusion] Add failing interface tests

[tobias-tengler]

We noticed there's a similar issue with selections on interfaces, similarly to unions (#7686).

For example the following query doesn't work if there is more than one type implementing the interface, as all query plan steps will reference the same selection set ({ name }) to use for synchronization, leading to issues:

{
  interfaceField {
    product {
      name # resolved from different subgraph
    }
  }
}

This PR adds a bunch of tests for interface cases and marks the failing ones with [Fact(Skip = "...")].

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (73914c6) to head (cbe521f).
Report is 13 commits behind head on main-version-15.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           main-version-15   #8272       +/-   ##
===================================================
- Coverage            74.89%       0   -74.90%     
===================================================
  Files                 2937       0     -2937     
  Lines               155047       0   -155047     
  Branches             18177       0    -18177     
===================================================
- Hits                116122       0   -116122     
+ Misses               32875       0    -32875     
+ Partials              6050       0     -6050     

Flag	Coverage Δ
unittests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.