Re-assess output shown in elastic-package when running tests in CI

[mrodm]

Currently, the output of elastic-package when running tests is quite verbose and sometimes it becomes difficult to find the errors or the information that we need.
Example of current builds:
https://buildkite.com/elastic/integrations/builds/12479#019009c4-26f7-4841-a76b-0750d8a6c2a6/1800-1801

It should be re-assessed which messages are written as logs (and which log level) as well as which messages should be written directly to the stdout/stderr to the user (fmt.Printf).

It could be checked to introduce a new log level (e.g. TRACE) to avoid writing to the output some of the current messages.

It is needed to be sure that any error raised during the CI build is also visible easily without having to go through all the logs.

Related PR #1876

• Used log/slog package for structured logging
• Added new log level, adding for instance -vvv