Pkg.test() drops Preferences

[staticfloat]

While attempting to move FFTW over to using Preferences instead of environment variables/mutating its own state to record which backend is currently in use, I have discovered that Pkg generates a new project when running tests, and that new project drops unknown metadata. I believe the culprit is this section:

Pkg.jl/src/Operations.jl

Lines 1516 to 1534 in 0fe89b1

	# collect relevant info from source
	source_env = EnvCache(projectfile_path(source_path))
	# collect regular dependencies
	test_project.deps = source_env.project.deps
	# collect test dependencies
	for name in get(source_env.project.targets, target, String[])
	uuid = get(source_env.project.extras, name, nothing)
	if uuid === nothing
	pkgerror("`$name` declared as a `$target` dependency, but no such entry in `extras`")
	end
	test_project.deps[name] = uuid
	end
	# collect compat entries
	for (name, uuid) in test_project.deps
	compat = get_compat_str(source_env.project, name)
	compat === nothing && continue
	set_compat(test_project, name, compat)
	end
	return test_project

Since users (and other packages) are storing pieces of information inside of Project.toml files, I think we need to preserve as much information in the Project.toml as possible.

For preferences in particular, we also need to copy any LocalPreferences.toml files to the temporary directory where the project is stored as well. I am currently working around this by generating an explicit test/Project.toml, but this is obviously sub-optimal.

[simonbyrne]

Same for Pkg.build.

[vchuravy]

We might want to do this for the sandbox used for .pkg/select_artifacts.jl as well.

[vchuravy]

It seems like adding the preferences to the test/Project.toml instead of test/LocalPreferences.toml works on 1.7

[deps]
Preferences = "21216c6a-2e73-6563-6e65-726566657250"
Foo = "48898bec-3adb-11e9-02a6-a164ba74aeae"

[preferences]
Foo.tree = "birch"