Parallelise unit tests with ParallelTestRunner

[Alexsp32]

Instead of splitting unit tests into groups based on the ordering in runtests.jl, have multiple parallel workers execute every separate test file instead. This reduces setup overhead and hopefully should make things slightly faster.

Strategy for future unit tests.

• When adding unit tests, make sure to add any new package dependencies to test/Project.toml, not the main Project.toml.
• Unit tests should be as small as practical, as they will be run in parallel automatically.
• This means no include() statements in test sets.
• Any files in test/ will be read automatically, no need to add anything more to test/runtests.jl.

[htsnowden]

This looks good to me. Should there be something added to the developer docs page to cover how to do tests