cdecl: add test

Closes Homebrew/homebrew#26416.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Formula/cdecl.rb

@@ -21,4 +21,10 @@ class Cdecl < Formula
                    "MANDIR=#{man1}",
                    "install"
   end
+
+  test do
+    output = `#{bin}/cdecl explain int *a`.strip
+    assert_equal "declare a as pointer to int", output
+    assert_equal 0, $?.exitstatus
+  end
 end