Switch tests to use to `TestItems.jl` for easier integration with IDEs and other advanced tests runners

Like any modern language, Julia supports running unit tests as part of the dev workflow. In this package they are in the `test` folder and can be executed by going to package mode and running `] test QuantumOptics`. They are build with the `Test` standard library. There are testing harness libraries that have been build on top of it, providing many additional features. `TestItems` in particular makes it much easier to run tests in parallel, it integrates with various IDEs, and provides other nice features. ### This bounty is about packaging all of the tests in QuantumOptics and QuantumOpticsBase as `TestItems` tests. The conversion is fairly straightforward: For each test file, wrap it in `@testitem "description" ... end`. It would be best to **not** change any other indentation in that file, so that the diffs shown in the eventual PR are simple and easy to review. Modify the `runtests.jl` file to be much simpler than it currently is, and to use the `TestItems` runner. The Project.toml file will need minor modifications to add the test dependencies on `TestItems` You can consult other libraries in this same ecosystem to see how straightforward the change is, e.g. https://github.com/QuantumSavory/QuantumClifford.jl/tree/master/test To complete this bounty: - A PR for QuantumOptics that performs the aforementioned conversion and has all tests passing and no lost tests - A PR for QuantumOpticsBase that performs the aforementioned conversion and has all tests passing and no lost tests - there might be some minor back-and-forth on editing ENV variables to make sure that tests are filtered properly by the CI runners (see how it is done in QuantumClifford.jl) - both PRs need to be approved and merged for the bounty to be awarded If you are new to julia, make sure to: - install julia using `juliaup` for version management - look through the [julia manual](https://docs.julialang.org/en/v1/) and [modern workflows](https://modernjuliaworkflows.org/) - try out VSCode with the [Julia plugin](https://www.julia-vscode.org/) for good IDE support