Renames assertions for consistency

[nkakouros]

One assertion is named (notice the final s):

assert_file_contains

and another (notice the missing s):

assert_file_exist

This is confusing. I suggest that all assertions have a final s where it makes sense. E.g. assert_file_exists.

Since existing project are using the old type without the final s, aliases are created for those old versions.

[martin-schulze-vireso]

I think if we want to move forward with this, we should at least not break existing code. We should offer aliases with the old names for backwards compatibility.

Can you add this and rebase onto current master to get the tests green?

[martin-schulze-vireso]

After closer inspection a saw that you already had aliases in place. I must have scrolled past them. I tried force push a rebased version but your remote denied access. Can you do so yourself or grant access?

[martin-schulze-vireso]

Thanks for the PR and your fast response. I had to force the merge as your commit was not signed. Please consider signing your commits if you have a PGP/GnuPG key or allow changes by the maintainers on your future PRs, so we can sign for you.