Disable testing properly

[ablaom]

Wasted some time trying to understand what test/runtests.jl actually does (after recovering a functioning Project.toml) and why CI.yml is disabled (commented out).

Here's the reason, from a comment in /test/runtests.jl added in this PR:

# This file was originally developed to test that each tutorial.jl file could be
# executed. When written, all tutorials were based on a *single* Manifest.toml, appearing
# at the top level. But now each tutorial get's it's own environment and the approach
# taken here does not work.

After this PR, you can now run Pkg.test("DataScienceTutorials") without weird output, but it executes no actual testing and prints a warning explaining the current situation.

Will open PR shortly to flag the lack of automated testing.

This PR also adds /Manifest.toml to .gitignore and removes this top level Manifest.toml, which is not attached to any tutorial, but used only locally for developers (e.g., for include("deploy.jl")).

[EssamWisam]

Looks good. Thanks.