test: make build fail if NDEBUG is defined

Defining NDEBUG via CFLAGS is the canonical way to disable assertions in
C. mpv respects this (and ta.c actually disables some debugging
machinery if it's defined).

But for tests, this is not useful at all. So if --enable-tests is passed
to configure, the user must not define NDEBUG, even if the rest of the
player does not care.

(We could just #undef NDEBUG, but let's not. Tests calling into the rest
of the player might depend on asserts there, or so.)

test/index.c

@@ -51,3 +51,7 @@ bool run_tests(struct MPContext *mpctx)
 
     return num_run > 0; // still error if none
 }
+
+#ifdef NDEBUG
+static_assert(false, "don't define NDEBUG for tests");
+#endif