Add tests.

test/CMakeLists.txt

@@ -29,6 +29,10 @@ expect_compile_error("fmt::internal::Array<char, 5> a, b; b = a;")
 expect_compile_error("fmt::Writer a, b(a);")
 expect_compile_error("fmt::Writer a, b; b = a;")
 
+# Formatter is not copyable from a temporary.
+expect_compile_error("fmt::Formatter<> a(fmt::Formatter<>(\"a\"));")
+expect_compile_error("fmt::Formatter<> b(\"a\"); b = fmt::Formatter<>(\"b\");")
+
 # Writing a wide character to a character stream Writer is forbidden.
 expect_compile_error("fmt::Writer() << L'a';")
 expect_compile_error("fmt::Writer() << fmt::pad(\"abc\", 5, L' ');")