oasis-core/go/oasis-test-runner: Add observer fixture and e2e test

[martintomazic]

Closes #6495

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[martintomazic]

SGX tests seems to be failing because of the CI runner issue: see

Update: was fixed.

[martintomazic]

To be able to run this in non-sgx mode we would have to fix:
https://github.com/oasisprotocol/oasis-core/blob/master/go/worker/keymanager/secrets.go#L245:L264. I.e. in non-sgx mode access list obtained from the consensus registered nodes is not applied.

Imo out of scope, also fine with stronger test albeit less practical to test locally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.60%. Comparing base (4ab1cc1) to head (f09088e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6497      +/-   ##
==========================================
+ Coverage   64.52%   64.60%   +0.08%     
==========================================
  Files         698      698              
  Lines       68208    68220      +12     
==========================================
+ Hits        44012    44075      +63     
+ Misses      19139    19095      -44     
+ Partials     5057     5050       -7     

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[peternose]

LGTM 👍

[peternose]

The file .changelog/6495.trivial.md could also be renamed to .changelog/6497.trivial.md, to point to PR not the issue. As an issue could have multiple PRs, with different commits which are seen in the changelog.