`search_settable_param` fix for finding wrong parameters

Explanation of changes

When the prepare function is called on a Component, it sets qcodes parameters from the settings. From the hardcoded qcodes parameter names it finds the actual parameter, and sets them. However, there is a small issue with nested parameter names in the current code. Fortunately, it does not cause any issues currently, but in theory but it could.

The new test succeeded before this fix, even if there is no missing.ch_foo.bar parameter, only ch_foo.bar.

Motivation of changes

Fixing a potential issue.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (main for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• CHANGELOG.md and AUTHORS.md have been updated (when applicable).
• CI pipelines pass
  • pre-commit run --all-files --hook-stage commit passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds successfully (CI and readthedocs),
  • windows tests pass (manually triggered by maintainers before merging).

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