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](https://github.com/Homebrew/brew/issues/16708#issuecomment-1953483970).

Usage:

```
❯ git diff
diff --git a/Library/Homebrew/test/migrator_spec.rb b/Library/Homebrew/test/migrator_spec.rb
index 87fadd5e95..db4700810a 100644
--- a/Library/Homebrew/test/migrator_spec.rb
+++ b/Library/Homebrew/test/migrator_spec.rb
@@ -69,6 +69,8 @@ RSpec.describe Migrator do
       tab.source["tap"] = "homebrew/core"
       tab.write

+      binding.break
+
       expect do
         described_class.new(new_formula, "oldname")
       end.to raise_error(Migrator::MigratorDifferentTapsError)

issyl0 at pictor in /opt/homebrew on bye-byebug
❯ brew tests --only=migrator --debug
Randomized with seed 59158
1 process for 1 spec, ~ 1 spec per process
.==> Relinking newname
.==> Unlinking oldname
...==> Moving oldname versions to /private/tmp/homebrew-tests-20240403-85464-3uogqr/cellar/newname
....==> Migrating formula oldname to newname
==> Unlinking oldname
==> Moving oldname versions to /private/tmp/homebrew-tests-20240403-85464-3uogqr/cellar/newname
==> Relinking newname
....[67, 76] in ~/migrator_spec.rb
    67|       tab = Tab.empty
    68|       tab.tabfile = HOMEBREW_CELLAR/"oldname/0.1/INSTALL_RECEIPT.json"
    69|       tab.source["tap"] = "homebrew/core"
    70|       tab.write
    71|
=>  72|       binding.break
    73|
    74|       expect do
    75|         described_class.new(new_formula, "oldname")
    76|       end.to raise_error(Migrator::MigratorDifferentTapsError)
=>#0    block in <top (required)> (3 levels) at ~/migrator_spec.rb:72
  #1    [C] BasicObject#instance_exec at /opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/gems/rspec-core-3.13.0/lib/rspec/core/example.rb:263
  # and 68 frames (use `bt' command for all frames)
(rdbg@/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/bin/rspec#85464) p tab
(rdbg@/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/bin/rspec#85464) p    # command(rdbg@/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/bin/rspec#85464) p     # command t    # command ta    # command tab    # command(rdbg@/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/bin/rspec#85464) p tab    # command
=> #<Tab:0x0000000107156be0 @aliases=[], @arch=nil, @built_as_bottle=false, @built_on={"os"=>"Macintosh", "os_version"=>"macOS 14", "cpu_family"=>"arm_firestorm_icestorm"}, @compiler=:clang, @homebrew_version="4.2.16-55-gc8f60ec-dirty", @installed_as_dependency=false, @installed_on_request=false, @loaded_from_api=false, @poured_from_bottle=false, @runtime_dependencies=nil, @source={"path"=>nil, "tap"=>"homebrew/core", "tap_git_head"=>nil, "spec"=>"stable", "versions"=>{"stable"=>nil, "head"=>nil, "version_scheme"=>0}}, @source_modified_time=0, @stdlib=nil, @tabfile=#<Pathname:/private/tmp/homebrew-tests-20240403-85464-3uogqr/cellar/oldname/0.1/INSTALL_RECEIPT.json>, @time=nil, @unused_options=[], @used_options=[]>
(rdbg@/opt/homebrew/Library/Homebrew/vendor/bundle/ruby/3.1.0/bin/rspec#85464)
```

.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 @@ module Homebrew
         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 @@ module Homebrew
         # 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/retry"
 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 @@ RSpec.configure do |config|
     @__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)