Add singular form of spec metrics

[ItsASine]

sec and secs, min and mins, hour and hours

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The specs all pass, though I will admit that I'm not good enough with regex to optimize it to match sec or secs but not second in any way but (?!second)secs?

That replace in test-helper was changing the spec display to be {test}ond in 3 places, like 1.1.2 {test}ond grandchild suite, so the specs were failing without the (?!second)

[bcaudan]

Thanks a lot for your help!

I made some refactorings:

• 971e392: I prefer to make two regex to not fall into that "second" edge case
• 5ed13b4: I extract a method for the pluralization logic

It is available in v2.4.0.

[ItsASine]

Looks good, thanks 👍