Test folder for files without a language

This pull request adds, in `test/fixtures/`, a new special folder for data files (ie. files for which Linguist shouldn't find a language). It also adds a new test (pchaigno/linguist@7744428) to check that files in this folder don't have a language.

The second commit moves the folders `samples/Binary/` and `samples/Text/` into `test/fixtures/`.
Files in this folders were only used as test files, thus should be in `fixtures`.

This pull request will allow us to quickly add new tests for files that shouldn't be recognized by Linguist. It should help prevent bugs such as #1808.