fuzz: [refactor] Use 100'000 digit separator in __AFL_LOOP

This makes it easier to glance the exact value.

src/test/fuzz/fuzz.cpp

@@ -231,7 +231,7 @@ int main(int argc, char** argv)
     // Enable AFL persistent mode. Requires compilation using afl-clang-fast++.
     // See fuzzing.md for details.
     const uint8_t* buffer = __AFL_FUZZ_TESTCASE_BUF;
-    while (__AFL_LOOP(100000)) {
+    while (__AFL_LOOP(100'000)) {
         size_t buffer_len = __AFL_FUZZ_TESTCASE_LEN;
         test_one_input({buffer, buffer_len});
     }