tests: don't output seed multiple times.

This clutters up the output. Instead, hide it with a RSpec formatter and
generate and output it ourselves.

Library/Homebrew/dev-cmd/tests.rb

@@ -82,17 +82,20 @@ module Homebrew
         ]
       end
 
+      # Generate seed ourselves and output later to avoid multiple different
+      # seeds being output when running parallel tests.
+      seed = ARGV.include?("--seed") ? ARGV.next : rand(0xFFFF).to_i
+
       args = ["-I", HOMEBREW_LIBRARY_PATH/"test"]
       args += %W[
+        --seed #{seed}
         --color
         --require spec_helper
-        --format progress
+        --format NoSeedProgressFormatter
         --format ParallelTests::RSpec::RuntimeLogger
         --out #{HOMEBREW_CACHE}/tests/parallel_runtime_rspec.log
       ]
 
-      args << "--seed" << ARGV.next if ARGV.include? "--seed"
-
       unless OS.mac?
         args << "--tag" << "~needs_macos"
         files = files.reject { |p| p =~ %r{^test/(os/mac|cask)(/.*|_spec\.rb)$} }
@@ -102,6 +105,8 @@ module Homebrew
         files = files.reject { |p| p =~ %r{^test/os/linux(/.*|_spec\.rb)$} }
       end
 
+      puts "Randomized with seed #{seed}"
+
       if parallel
         system "bundle", "exec", "parallel_rspec", *opts, "--", *args, "--", *files
       else

Library/Homebrew/test/spec_helper.rb

@@ -5,6 +5,7 @@ require "rspec/wait"
 require "rubocop"
 require "rubocop/rspec/support"
 require "set"
+require "support/no_seed_progress_formatter"
 
 if ENV["HOMEBREW_TESTS_COVERAGE"]
   require "simplecov"

Library/Homebrew/test/support/no_seed_progress_formatter.rb

@@ -0,0 +1,7 @@
+require "rspec/core/formatters/progress_formatter"
+
+class NoSeedProgressFormatter < RSpec::Core::Formatters::ProgressFormatter
+  RSpec::Core::Formatters.register self, :seed
+
+  def seed(notification); end
+end