Fix application uplink queue test flake

[adriansmares]

Summary

References https://github.com/TheThingsIndustries/lorawan-stack/pull/3890

This PR fixes this flake.

Changes

• Insert the uplinks into the uplink queue before attempting to read them.
• Append uplinks synchronously, instead of using a goroutine (which we are not waiting).

Testing

go test -v -count=500 -run=TestApplicationUplinkQueue
...
PASS
ok      go.thethings.network/lorawan-stack/v3/pkg/networkserver/redis   32.394s

No manual testing is required. This PR strictly treats a testing flake.

Regressions

N/A.

Notes for Reviewers

@cvetkovski98 please take a look.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[cvetkovski98]

Reproduced the flake locally by running the tests with -count 10. The change fixes the problem.