[TEST] Added some tests

[eleanorfrajka]

Description

Initial add of tests for plotters, merge_datasets.py and tools.py.

Checklist

• I have updated or added tests if needed
• I have updated the documentation if needed
• I have run the code and verified it works
• I have run pre-commit run --all-files (if pre-commit is installed)

[Copilot]

Pull Request Overview

This PR adds initial test coverage for three key modules in the WBTSdata package: plotters, merge_datasets, and tools. The tests include unit tests, edge case handling, and integration test placeholders to establish a testing foundation for the codebase.

• Added comprehensive test suites for tools.py (config loading and unit conversion functions)
• Added test coverage for plotters.py (plotting functions with matplotlib testing patterns)
• Added test structure for merge_datasets.py (directory listing and dataset merging functions)

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_tools.py	Unit tests for config loading and unit conversion functionality
tests/test_plotters.py	Tests for plotting functions including edge cases and visual tests
tests/test_merge_datasets.py	Tests for dataset merging and directory discovery functions
tests/conftest.py	Pytest fixtures providing sample datasets and configuration for testing
requirements-dev.txt	Added pytest dependencies for testing infrastructure
pyproject.toml	Added pytest configuration with markers and test options
WBTSdata/plotters.py	Minor fix to ensure y-tick positions are set before labels

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