(maint) Refactor file content remote source specs

The "from an explicit fileserver" describe block mostly duplicted the original
"from remote source" block and so the two have been merged. Additionally, some
specific improvements have been made to the tests:

• Tests for puppet:/// vs. puppet:// have been fixed and moved to explicit scenarios rather than implicit expectations in the global before block
• The number of lines written to test files has been reduced from 10000 to 10. This speeds up the suite and prevents failing tests from belching a wall of red text.
• Replace instances of .should with expect().to.