[NO-TICKET] Fix failing "Ruby gem scaffolding" CI (#1153)

* Fix specs failing due to slightly changed behavior on ruby-master

Some of our tests use

```ruby
let(:temporary_directory) { "does/not/exist" }
```

and in ruby/fileutils#132 the behavior of
`FileUtils.remove_dir` was tweaked to change the exception that gets
raised when doing `FileUtils.remove_dir` with a directory that doesn't
exist.

I've updated the test clean-up hook with the new exception as well.

* Minor: Add `irb` to development dependencies

May come in handy sometimes (and does not affect what we ship to
customers).

* Minor: Refresh rspec configuration

As per https://ivoanjo.me/blog/2016/09/25/why-you-should-regenerate-your-spec-helper/

Author: ivoanjo

ruby/gems.rb

@@ -11,3 +11,4 @@
 gem "pry"
 gem "pry-byebug" unless RUBY_VERSION > "3.1"
 gem "rubygems-await" unless RUBY_VERSION < "3.1"
+gem "irb"

ruby/spec/libdatadog_spec.rb

@@ -25,7 +25,7 @@
     after do
       begin
         FileUtils.remove_dir(temporary_directory)
-      rescue Errno::ENOENT => _e
+      rescue Errno::ENOENT, Errno::ENOTDIR
         # Do nothing, it's ok
       end
     end

ruby/spec/spec_helper.rb

@@ -4,14 +4,66 @@
 
 require "libdatadog"
 
+# This file was generated by the `rspec --init` command + manually tweaked.
+#
+# See https://rubydoc.info/gems/rspec-core/RSpec/Core/Configuration
 RSpec.configure do |config|
-  # Enable flags like --only-failures and --next-failure
+  config.expect_with :rspec do |expectations|
+    # This option will default to `true` in RSpec 4.
+    expectations.include_chain_clauses_in_custom_matcher_descriptions = true
+  end
+
+  config.mock_with :rspec do |mocks|
+    # This will default to `true` in RSpec 4.
+    mocks.verify_partial_doubles = true
+  end
+
+  # This option will default to `:apply_to_host_groups` in RSpec 4.
+  config.shared_context_metadata_behavior = :apply_to_host_groups
+
+  # This allows you to limit a spec run to individual examples or groups
+  # you care about by tagging them with `:focus` metadata. When nothing
+  # is tagged with `:focus`, all examples get run. RSpec also provides
+  # aliases for `it`, `describe`, and `context` that include `:focus`
+  # metadata: `fit`, `fdescribe` and `fcontext`, respectively.
+  config.filter_run_when_matching :focus
+
+  # Allows RSpec to persist some state between runs in order to support
+  # the `--only-failures` and `--next-failure` CLI options. We recommend
+  # you configure your source control system to ignore this file.
   config.example_status_persistence_file_path = ".rspec_status"
 
   # Disable RSpec exposing methods globally on `Module` and `main`
   config.disable_monkey_patching!
 
-  config.expect_with :rspec do |c|
-    c.syntax = :expect
+  # This setting enables warnings. It's recommended, but in some cases may
+  # be too noisy due to issues in dependencies.
+  config.warnings = true
+
+  # Many RSpec users commonly either run the entire suite or an individual
+  # file, and it's useful to allow more verbose output when running an
+  # individual spec file.
+  if config.files_to_run.one?
+    # Use the documentation formatter for detailed output,
+    # unless a formatter has already been configured
+    # (e.g. via a command-line flag).
+    config.default_formatter = "doc"
   end
+
+  # Print the 10 slowest examples and example groups at the
+  # end of the spec run, to help surface which specs are running
+  # particularly slow.
+  config.profile_examples = 10
+
+  # Run specs in random order to surface order dependencies. If you find an
+  # order dependency and want to debug it, you can fix the order by providing
+  # the seed, which is printed after each run.
+  #     --seed 1234
+  config.order = :random
+
+  # Seed global randomization in this process using the `--seed` CLI option.
+  # Setting this allows you to use `--seed` to deterministically reproduce
+  # test failures related to randomization by passing the same `--seed` value
+  # as the one that triggered the failure.
+  Kernel.srand config.seed
 end