avfilter/vsrc_mptestsrc: Change enum to int, which is accessed via AVOption as int

This fixes depending on implementation defined behavior
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/vsrc_mptestsrc.c

@@ -55,7 +55,7 @@ typedef struct MPTestContext {
     AVRational frame_rate;
     int64_t pts, max_pts, duration;
     int hsub, vsub;
-    enum test_type test;
+    int test;           ///< test_type
 } MPTestContext;
 
 #define OFFSET(x) offsetof(MPTestContext, x)