Add Test as a real dependency

[ffevotte]

TestSetExtensions currently lists Test as a test-only dependency, which causes  Pkg to complain whenever TestSetExtensions is used in other contexts than its own tests:

julia> using TestSetExtensions
┌ Warning: Package TestSetExtensions does not have Test in its dependencies:
│ - If you have TestSetExtensions checked out for development and have
│   added Test as a dependency but haven't updated your primary
│   environment's manifest file, try `Pkg.resolve()`.
│ - Otherwise you may need to report an issue with TestSetExtensions
└ Loading Test into TestSetExtensions from project dependency, future warnings for TestSetExtensions are suppressed.

This PR fixes that by simply declaring Test as a standard dependency of TestSetExtensions.

Thanks for this package, and please keep up the good work!

[ffevotte]

FWIW, it looks like the AppVeyor tests failure is completely unrelated to the changes proposed in this PR.