[dev] Test replies: encrypt a string instead of 1 random byte

[redshiftzero]

Status

Ready for review

Description of Changes

Changes proposed in this pull request:

• Replaces the 1 random byte message with a nice test reply string
• This was the source of confusion over in Downlod replies securedrop-client#114

Testing

make -C securedrop dev

Deployment

Dev environment only

Checklist

If you made changes to the server application code:

• Linting (make ci-lint) and tests (make -C securedrop test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally

[codecov-io]

Codecov Report

Merging #3927 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #3927   +/-   ##
========================================
  Coverage    84.55%   84.55%           
========================================
  Files           43       43           
  Lines         2726     2726           
  Branches       296      296           
========================================
  Hits          2305     2305           
  Misses         354      354           
  Partials        67       67

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