tests: allow passing --seed <number>

This parameter is printed by every `brew tests` run and feeding it back
into the test suite allows to get an identical run. Make it easier to
do that by accepting the argument as it is printed during `brew tests`.

This requires a slight adjustment to named-argument passing as the
number that follows `--seed` is also seen as a named argument, but we
usually only want to pass `TEST=<something>` and `TESTOPTS=<something>`.

Library/Homebrew/cmd/tests.rb

@@ -22,9 +22,12 @@ module Homebrew
         system "bundle", "install", "--path", "vendor/bundle"
       end
 
+      # Make it easier to reproduce test runs.
+      ENV["SEED"] = ARGV.next if ARGV.include? "--seed"
+
       args = []
       args << "--trace" if ARGV.include? "--trace"
-      args += ARGV.named
+      args += ARGV.named.select { |v| v[/^TEST(OPTS)?=/] }
       system "bundle", "exec", "rake", "test", *args
 
       Homebrew.failed = !$?.success?