Eliminate use of empty struct in test code

Where GCC and Clang issue warnings such as "empty struct is a GNU extension"
(with `-Wgnu-empty-struct`) or "empty struct has size 0 in C, size 1 in C++"
(with `-Wc++-compat`), MSVC issues an error C2016 "C requires that a struct
or union has at least one member".

mytests.c

@@ -54,7 +54,9 @@ CTEST2(memtest, test2) {
 }
 
 
-CTEST_DATA(fail) {};
+CTEST_DATA(fail) {
+    int unused;
+};
 
 // Asserts can also be used in setup/teardown functions
 CTEST_SETUP(fail) {