add spec for CLI option "-W:no-deprecated" introduced in Ruby 2.7

[lxxxvi]

Hello 👋

Here's my suggestion to cover this one from #745

The -W option has been extended with a following :, to manage categorized
warnings. Feature #16345 Feature #16420

I always find it tricky to assert for nothing (empty string), that's why I also added an assertion for the deprecation warning if option -W:no-deprecated is not given. Do you think this makes sense?

Looking forward to receiving your feedback.

[eregon]

Thanks, this looks good.
I'm not sure there is a good way to emit a deprecation warning no matter the Ruby version running the test.

[andrykonchin]

TBH I am wondering how '2>&1' works on Windows. AFAIK we run specs on CI on Windows as well.

[eregon]

I think whatever shell is used by Ruby on Windows supports it. @MSP-Greg probably knows better.

[MSP-Greg]

2>&1 works in PowerShell. I use it building ruby-loco.