tests: add spread suite for nil plugin

[kyrofa]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

---

This PR resolves LP: #1783838 by moving the plugin integration tests for the Nil plugin into a Spread suite.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@1af1ae5). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2201   +/-   ##
========================================
  Coverage          ?   91.1%           
========================================
  Files             ?     201           
  Lines             ?   12924           
  Branches          ?    1915           
========================================
  Hits              ?   11775           
  Misses            ?     780           
  Partials          ?     369

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1af1ae5...c96caf3. Read the comment docs.

[sergiusens]

so complex!