Ensure that unavailable_nodes list is a subset of discovered_nodes

[arcusfelis]

Addresses "MIM-2054 CETS status: removed nodes from disco should not be listed in a list of unavailable nodes"

Changes:

• Ensure that unavailable_nodes list is a subset of discovered_nodes
• Add status_discovered_nodes testcase (a test for this field was missing)
• Use ordsets in types for nodes (because they are ordsets)
• Define DiscoOpts in tests instead of constructing the same map twice (small refactoring)

Nodes could be removed from the discovery_nodes table after some time.
After that point, the removed node should not appear in the systemInfo API.
But we currently we can have:

./_build/mim1/rel/mongooseim/bin/mongooseimctl cets systemInfo
{
  "data" : {
    "cets" : {
      "systemInfo" : {
        "unavailableNodes" : [
          "badnode@localhost"
        ],
        "discoveredNodes" : [
          "mongooseim2@localhost",
          "mongooseim3@localhost",
          "mongooseim@localhost"
        ],
        "availableNodes" : [
          "mongooseim@localhost",
          "mongooseim2@localhost",
          "mongooseim3@localhost"
        ]
        ....

We need to ensure that unavailableNodes is a subset of discoveredNodes before returning it to the user.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (def7da2) 97.38% compared to head (8fcd64b) 97.46%.
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   97.38%   97.46%   +0.07%     
==========================================
  Files           9        9              
  Lines         612      630      +18     
==========================================
+ Hits          596      614      +18     
  Misses         16       16              

Files	Coverage Δ
src/cets_discovery.erl	93.66% <100.00%> (+0.09%)	⬆️

... and 2 files with indirect coverage changes

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[chrzaszcz]

Looks good - I only added minor style-related comments.

[arcusfelis]

added meck

[chrzaszcz]

👌