Document two new ways of specifying test-specific dependencies

[mgkuhn]

The Test-specific dependencies section of the documentation currently only lists two older methods for specifying test-specific dependencies:

• [targets] + [extras] in main Project.toml
• test/Project.toml: add only additional dependencies there

Both of these older methods have been somewhat unsatisfactory: they only work via special magic applied by Pkg.test(), and they do not work if one simply tries to activate a local test environment with julia --project=test/ runtests.jl.

Fortunately there are now two new, alternative, better ways of solving this problem, which do not rely on special magic applied by Pkg.test(), and these finally allow the test folder to be just a normal project:

• [source] in test/Project.toml: under [deps] add ProjectName=UUID as a dependency and under [source] also add ProjectName={path='..'} such that the cd test/ && julia --project environment finds the project to test in its own parent folder
• [workspace] in Project.toml: add projects = ["test"] such that the test environment finds the project to test in its own parent folder

At the moment, these new features are still hidden in the Project.toml format description

It would be great to have these documented as well under https://pkgdocs.julialang.org/dev/creating-packages/#Test-specific-dependencies