Make it possible to skip the fuzz test with an environment variable. (It's very long.)

c++/src/capnp/fuzz-test.c++

@@ -24,12 +24,24 @@
 #include "message.h"
 #include "serialize.h"
 #include <kj/test.h>
+#include <stdlib.h>
+#include <unistd.h>
 #include "test-util.h"
 
 namespace capnp {
 namespace _ {  // private
 namespace {
 
+struct SkipTestHack {
+  SkipTestHack() {
+    if (getenv("CAPNP_SKIP_FUZZ_TEST") != nullptr) {
+      char message[] = "Skipping test because CAPNP_SKIP_FUZZ_TEST is set in environment.\n";
+      write(STDOUT_FILENO, message, sizeof(message));
+      _exit(0);
+    }
+  }
+} skipTestHack;
+
 uint64_t traverse(AnyPointer::Reader reader);
 uint64_t traverse(AnyStruct::Reader reader);
 uint64_t traverse(AnyList::Reader reader);