lavu/opt-test: do not explicitly set default value in test_ctx

Let av_opt_set_defaults() set the value, which is simpler and more
idiomatic.

libavutil/opt.c

@@ -1323,7 +1323,7 @@ static const AVOption test_options[]= {
 {"num",      "set num",        OFFSET(num),      AV_OPT_TYPE_INT,      {.i64 = 0},       0,        100                 },
 {"toggle",   "set toggle",     OFFSET(toggle),   AV_OPT_TYPE_INT,      {.i64 = 0},       0,        1                   },
 {"rational", "set rational",   OFFSET(rational), AV_OPT_TYPE_RATIONAL, {.dbl = 0},  0,        10                  },
-{"string",   "set string",     OFFSET(string),   AV_OPT_TYPE_STRING,   {0},              CHAR_MIN, CHAR_MAX            },
+{"string",   "set string",     OFFSET(string),   AV_OPT_TYPE_STRING,   {.str = "default"}, CHAR_MIN, CHAR_MAX          },
 {"flags",    "set flags",      OFFSET(flags),    AV_OPT_TYPE_FLAGS,    {.i64 = 0},       0,        INT_MAX, 0, "flags" },
 {"cool",     "set cool flag ", 0,                AV_OPT_TYPE_CONST,    {.i64 = TEST_FLAG_COOL}, INT_MIN,  INT_MAX, 0, "flags" },
 {"lame",     "set lame flag ", 0,                AV_OPT_TYPE_CONST,    {.i64 = TEST_FLAG_LAME}, INT_MIN,  INT_MAX, 0, "flags" },
@@ -1385,7 +1385,6 @@ int main(void)
 
         test_ctx.class = &test_class;
         av_opt_set_defaults(&test_ctx);
-        test_ctx.string = av_strdup("default");
 
         av_log_set_level(AV_LOG_DEBUG);