Add .gitignore to stdlib-fpm deployment for generated test files

[minhqdao]

Motivation

A number of .txt,.bin, .dat and .stream files are created when running tests with fpm test on the stdlib-fpm branch. However, a .gitignore file is missing so the files end up cluttering the git diff and need to be manually deleted every time. While most of the development work will be done from the master branch, I think it is useful to be able to switch to the stdlib-fpm branch to try things out.

Prior Art

There is a .gitignore file on the master(branch name also needs change?) branch so some entries can be copied over during deployment.

Additional Information

I believe it would be more advantageous to avoid placing any files generated by tests directly in the project's root directory. Instead, they could be stored, for instance, in a build/test or tmp/test folder (open for suggestions). However, I acknowledge that for the time being, adding a .gitignore file is the simplest solution.

[perazz]

The stdlib-fpm is generated on deployment @minhqdao, please check back the details at https://github.com/fortran-lang/stdlib/blob/master/ci/fpm-deployment.sh