chore(test): address flakiness of grpc logs test

[StarlightIbuki]

Summary

Let's clear the log before the test.

Checklist

• The Pull Request has tests
• There's an entry in the CHANGELOG
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• clear the log before the test.
• refactor to fail with more clear messages

Issue reference

Please remember to remove the issue reference from the commit message. It's added by mistake.

[StarlightIbuki]

FAILED  spec/03-plugins/03-http-log/01-log_spec.lua:444: Plugin: http-log (log) [#postgres] logs to HTTP (#grpcs)
./spec/helpers.lua:1683: Failed to assert eventual condition

This means, it really has encountered a situation where the log is not emitted.

The fix is still needed but it does not seem to fix the real issue.

My bad. Mistakenly checking grpc instead of grpcs.