Add rstest for testing

[trueleo]

Description

This PR introduces rstest as main unit testing framework. It's added as a development dependency to the project. maplit is added for its hashmap macro.

Core advantages of using rstest

• can save on some boilerplate code
• easier to add new test cases
• #[once] Fixture to make an object initialise once and be shared between all tests

Disadvatages of using rstest

• Complex struct initialisation might not be possible in a test case. This can still be a no-issue because we can always write normal rust tests for anything that can't be done using rstest.

What is changed

• tests added to metadata.rs
• tests converted in query.rs
• tests converted in utils.rs

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This PR has:

• been tested to ensure log ingestion and log query works.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviours.

[nitisht]

@de-sh any comments on the rstest use?