Ruby 3.1: Add specs for Thread::Backtrace.limit

[ytjmt]

This PR adds specs for Feature #17479

From:

• Write specs for new Ruby 3.1 features and changes #923

Thread::Backtrace

• Thread::Backtrace.limit, which returns the value to limit backtrace length set by --backtrace-limit command line option, is added. [Feature #17479]

I've confirmed the default value is -1.
https://docs.ruby-lang.org/en/master/Thread/Backtrace.html

The defalt is -1 which means unlimited backtraces.

[eregon]

Thank you!