Allow testing projects without a UUID

[cossio]

Currently if you try to Pkg.test a project without an UUID, it will throw this error:

ERROR: trying to test unnamed project

Even if the project has a name.

Is there any reason why the Project.toml needs to have an UUID if I want to test it?

[cossio]

Related: #1982

[AnHeuermann]

For me it would already be an improvement to make the error message more specific. I needed to search for a while until I noticed that the missing UUID was the reason for the error message.
I created a Julia project that is not a package using DrWatson, see JuliaDynamics/DrWatson.jl#67.

So for the missing UUID something like

ERROR: trying to test unnamed project (missing UUID in Project.toml)

would improve my experience.

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Also related to #1253.