Fix OPUS request URLs

[alexdewar]

Description

Since commit 72b6e851, the request URLs used by OPUSInterface have been broken (#574), which causes commands to fail (e.g. starting recording). Fix by creating the URL from a string rather than a QUrl. We had unit tests for this but they didn't catch it.

Fixes #574.

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] Documentation (non-breaking change that adds or improves the documentation)
• [ ] Optimisation (non-breaking, back-end change that speeds up the code)

Key checklist

• [x] Pre-commit hooks run successfully (pre-commit run -a)
• [x] All tests pass (pytest)
• [ ] The documentation builds without warnings (mkdocs build -s)
• [x] Check the GUI still works (if relevant)
• [x] Check the code works with actual hardware (if relevant)
• [ ] Check the pyinstaller-built executable works (if relevant)

Further checks

• [x] Code is commented, particularly in hard-to-understand areas
• [x] Tests have been added or an issue has been opened to tackle that in the future. (Indicate issue here: # (issue))

[alexdewar]

Sorry, it was a slightly rushed description. The problem was that we were using the setPath() method of the QUrl to set the path with a query string, e.g. setPath("cmd.htm?opusrsstart"), but the question mark gets escaped rather than passed through. You need to use the setQuery() method instead to set this part, but I decided it was easier to just go back to using simple strings. The reason the test didn't pick it up was because the test didn't look at the case where there was a query string. Actually, I'll remedy that :smile: