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

[ytjmt]

This PR adds specs for Feature #17479

From:

• https://github.com/ruby/spec/issues/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.