It seems to pass under rubygems 2.5.2.

Since it covers the whole range we support.

7222: Remove `add_development_dependency` from new gems r=deivid-rodriguez a=deivid-rodriguez

### What was the end-user problem that led to this PR?

The problem was that a lot of gems in the wild use `add_development_dependency` for their development dependencies, but using `Gemfile`'s for that is more useful.

### What was your diagnosis of the problem?

My diagnosis was that the current situation is due to the fact that bundler generates a gem skeleton that uses `add_development_dependency` by default.

### What is your fix for the problem, implemented in this PR?

My fix is to stop using `add_development_dependency` in generated gems, and instead use the Gemfile.

### Why did you choose this fix out of the possible options?

I chose this fix because it encourages better practices.


Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

7235: Spec improvements r=deivid-rodriguez a=deivid-rodriguez

This is a bunch of miscelaneous commits that fix and improve some stuff in our test suite. I could open separate PRs for them but they are so small that I figured I could compile them all on a single PR.

Co-authored-by: David Rodríguez <deivid.rodriguez@riseup.net>

1.0.0 is such a common output, and it was hiding a crash in the test.