Convert brew custom-external-command test to spec.

Library/Homebrew/test/cmd/custom-external-command_spec.rb

@@ -0,0 +1,21 @@
+describe "brew custom-external-command", :integration_test do
+  it "is supported" do
+    Dir.mktmpdir do |path|
+      path = Pathname.new(path)
+
+      cmd = "custom-external-command-#{rand}"
+      file = path/"brew-#{cmd}"
+
+      file.write <<-EOS.undent
+        #!/bin/sh
+        echo 'I am #{cmd}.'
+      EOS
+      FileUtils.chmod "+x", file
+
+      expect { brew cmd, "PATH" => "#{path}#{File::PATH_SEPARATOR}#{ENV["PATH"]}" }
+        .to output("I am #{cmd}.\n").to_stdout
+        .and not_to_output.to_stderr
+        .and be_a_success
+    end
+  end
+end

Library/Homebrew/test/custom_command_test.rb

@@ -1,18 +0,0 @@
-require "testing_env"
-
-class IntegrationCommandTestCustomCommand < IntegrationCommandTestCase
-  def test_custom_command
-    mktmpdir do |path|
-      cmd = "int-test-#{rand}"
-      file = "#{path}/brew-#{cmd}"
-
-      File.open(file, "w") do |f|
-        f.write "#!/bin/sh\necho 'I am #{cmd}'\n"
-      end
-      FileUtils.chmod 0777, file
-
-      assert_match "I am #{cmd}",
-        cmd(cmd, "PATH" => "#{path}#{File::PATH_SEPARATOR}#{ENV["PATH"]}")
-    end
-  end
-end