Convert os/mac/blacklist test to spec.

Library/Homebrew/test/blacklist_spec.rb

@@ -1,12 +1,8 @@
 require "blacklist"
 
-RSpec::Matchers.define :be_blacklisted do
-  match do |actual|
-    blacklisted?(actual)
-  end
-end
-
 describe "Blacklist" do
+  matcher(:be_blacklisted) { match(&method(:blacklisted?)) }
+
   context "rubygems" do
     %w[gem rubygem rubygems].each do |s|
       subject { s }
@@ -103,9 +99,17 @@ describe "Blacklist" do
     it { is_expected.to be_blacklisted }
   end
 
-  context "haskell_platform" do
+  context "haskell-platform" do
     subject { "haskell-platform" }
 
     it { is_expected.to be_blacklisted }
   end
+
+  context "xcode", :needs_macos do
+    %w[xcode Xcode].each do |s|
+      subject { s }
+
+      it { is_expected.to be_blacklisted }
+    end
+  end
 end

Library/Homebrew/test/os/mac/blacklist_test.rb

@@ -1,12 +0,0 @@
-require "testing_env"
-require "blacklist"
-
-class OSMacBlacklistTests < Homebrew::TestCase
-  def assert_blacklisted(s)
-    assert blacklisted?(s), "'#{s}' should be blacklisted"
-  end
-
-  def test_xcode
-    %w[xcode Xcode].each { |s| assert_blacklisted s }
-  end
-end