add_subdirectory-test -> add-subdirectory-test for consistency

test/add-subdirectory-test/main.cpp

@@ -0,0 +1,8 @@
+#include "cppformat/format.h"
+
+int main(int argc, char** argv)
+{
+  for(int i = 0; i < argc; ++i)
+    fmt::print("{}: {}\n", i, argv[i]);
+  return 0;
+}