[Test] Fix race condition in test_build_image caused by export-image-logs and facilitate troubleshooting

[gmarciani]

Description of changes

Fix race condition in test_build_image caused by export-image-logs

PR #7486 traded one race for another.
It fixed export-image-logs running too late, when the log group has already been deleted as part of a successful
build-image teardown, by moving the export earlier. But it then ran too early: it only waited for the build log stream to exist, however the log stream must contain some events for the export task to produce something.

The fix waits for the build log stream to be non-empty, not just to exist, before exporting, so the log data is ready to be exported.

Also set explicit messages on the soft assertions used by the test to ease troubleshooting.

Tests

SUCCESS

test-suites:
  createami:
    test_createami.py::test_build_image:
      dimensions:
      - instances:
        - g4dn.2xlarge
        oss:
        - ubuntu2404
        regions:
        - eu-west-2
        schedulers:
        - slurm
      - instances:
        - g4dn.2xlarge
        oss:
        - alinux2
        regions:
        - eu-west-2
        schedulers:
        - slurm
      - instances:
        - g4dn.2xlarge
        oss:
        - rhel9
        regions:
        - ap-northeast-2
        schedulers:
        - slurm
      - instances:
        - g4dn.2xlarge
        oss:
        - ubuntu2204
        regions:
        - eu-west-2
        schedulers:
        - slurm
      - instances:
        - g4dn.2xlarge
        oss:
        - rhel8
        regions:
        - ap-northeast-2
        schedulers:
        - slurm

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.