Fix SyntaxError: unexpected keyword_rescue, expecting keyword_end in Ruby 2.4 and earlier

[taketo1113]

Background

A SyntaxError (syntax error, unexpected keyword_rescue, expecting keyword_end) occurs in the ipaddr gem's CI when running tests with Ruby 2.4:

Run bundle exec rake test
...
/home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/core_assertions.rb:75:in `require_relative': /home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/envutil.rb:108: syntax error, unexpected keyword_rescue, expecting keyword_end (SyntaxError)
      rescue Timeout::Error
            ^
/home/runner/work/ipaddr/ipaddr/vendor/bundle/ruby/2.4.0/gems/test-unit-ruby-core-1.0.9/lib/envutil.rb:481: syntax error, unexpected keyword_end, expecting end-of-input
...

https://github.com/ruby/ipaddr/actions/runs/18297568280/job/52098942424?pr=88#step:4:16

This error occurs only with the combination of Ruby 2.4 and test-unit-ruby-core 1.0.9.
It does not occur on Ruby 2.5 or later.

Details

In Ruby 2.4 and earlier, rescue cannot be used without an explicit begin/end block.
Therefore, this Pull Request adds the missing begin and end to fix the syntax error.

[hsbt]

Thanks always ❤️