Set up the debug gem for test debugging

- This processed that we'd requested a debugger, but didn't drop us into
  a debugging console until I
  [stopped the stdin disablement](#16708 (comment)).

.gitignore

@@ -21,6 +21,7 @@
 /Library/Taps
 /Library/PinnedTaps
 /Library/Homebrew/.byebug_history
+/Library/Homebrew/test/.rdbg_history
 
 # Ignore Bundler files
 **/.bundle/bin

Library/Homebrew/dev-cmd/tests.rb

@@ -21,6 +21,8 @@ class Tests < AbstractCommand
         switch "--online",
                description: "Include tests that use the GitHub API and tests that use any of the taps for " \
                             "official external commands."
+        switch "--debug",
+               description: "Enable debugging using ruby/debug."
         switch "--changed",
                description: "Only runs tests on files that were changed from the master branch."
         switch "--fail-fast",
@@ -43,6 +45,8 @@ def run
         # Given we might be testing various commands, we probably want everything (except sorbet-static)
         Homebrew.install_bundler_gems!(groups: Homebrew.valid_gem_groups - ["sorbet"])
 
+        require "debug" if args.debug?
+
         HOMEBREW_LIBRARY_PATH.cd do
           setup_environment!
 

Library/Homebrew/test/spec_helper.rb

@@ -29,6 +29,7 @@
 require "rspec/sorbet"
 require "rubocop/rspec/support"
 require "find"
+require "debug"
 require "timeout"
 
 $LOAD_PATH.unshift(File.expand_path("#{ENV.fetch("HOMEBREW_LIBRARY")}/Homebrew/test/support/lib"))
@@ -229,14 +230,14 @@
     @__stdin = $stdin.clone
 
     begin
-      if !example.metadata.keys.intersect?([:focus, :byebug]) && !ENV.key?("HOMEBREW_VERBOSE_TESTS")
+      if !example.metadata.keys.include?(:focus) && !ENV.key?("HOMEBREW_VERBOSE_TESTS")
         $stdout.reopen(File::NULL)
         $stderr.reopen(File::NULL)
+        $stdin.reopen(File::NULL)
       else
-        # don't retry when focusing/debugging
+        # don't retry when focusing
         config.default_retry_count = 0
       end
-      $stdin.reopen(File::NULL)
 
       begin
         timeout = example.metadata.fetch(:timeout, 60)