test: add default constructor for a const value

This fixed a compilation error of the OS X 10.11.6 C++ compiler:

    ./fmt/test/format-test.cc:1861:16: error: default initialization of an object of const type 'const Answer' without a user-provided default constructor
      const Answer const_answer;
                   ^
                               {}
diff --git a/test/format-test.cc b/test/format-test.cc
index 66d87c2..08828cc 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1917,7 +1917,7 @@
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), Answer()));
   Answer answer;
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), answer));
-  const Answer const_answer;
+  const Answer const_answer = Answer();
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), const_answer));
 }