Fixed duplicated output when writing in PRE_ALL report hook with a redirected output

samples/report.out.expected

@@ -1,5 +1,4 @@
 criterion_init
-criterion_init
 testing test in category sample
 Asserts: [1 passed, 1 failed, 2 total]
 criterion_fini

src/runner.c

@@ -296,6 +296,8 @@ static int criterion_run_all_tests_impl(void) {
     report(PRE_ALL, set);
     log(pre_all, set);
 
+    fflush(NULL); // flush everything before forking
+
     smart struct criterion_global_stats *stats = stats_init();
     map_tests(set, stats, run_test);