Allow definition of parameterized tests by subclassing TestSuite

[jamesmyatt]

The main updates are to allow parameterized tests to be defined by subclassing TestSuite.
Some other minor updates such as extracting functions from TestRunDisplay into utilities directory. Note that this directory also needs to be added on the path.

Example of parametrized tests in tests\helper_classes\TestParameterizedSuite.

[psexton]

Going to split this into multiple merges because this is such a large pull request:

13a. Change TestRunLogger.appendToLog from private to protected
13b. Extract display functions from TestRunDisplay
13c. Display extra information for MultipleErrors exception
13d. Move xml_io_tools
13e. Update tests readme (good catch on an munit mention I missed!)
13f. Add TestSuite subclassing

[psexton]

13a: 👍

[psexton]

13b: 👍 📝 Moved new functions from src/utilities to src/+xunit
13c: 👍

[psexton]

13d: 👎 Seems unnecessary to move them, unless we're moving them into a package and taking them out of the global namespace. (Which would be good, but would require bumping the semver to 5.0.0.)

[psexton]

13e: 👍

[psexton]

13f: Will re-look at after merging #20.

[jamesmyatt]

I've rebased these changes so that this now only contains those listed as (13f) in #13 (comment)

[psexton]

Is there really a need to support the old folder-based class system?

[psexton]

Rest of it looks fine, and tests pass as far back as I have installed.

[jamesmyatt]

Re: @-folders. These are still used a lot, including by Mathworks. So, I think that it's best to support it.

[jamesmyatt]

I'm going to close this and start a new one for just the TestSuite subclassing