Test for races in IptablesRestoreController::Init.

Bug: 28362720
Test: angler builds, boots
Test: netd_{unit,integration}_test pass
Change-Id: I73ed28c7e7edaeb65a3b346b89ec69f472fd5974
diff --git a/libnetdutils/Syscalls.cpp b/libnetdutils/Syscalls.cpp
index b0301d2..5354341 100644
--- a/libnetdutils/Syscalls.cpp
+++ b/libnetdutils/Syscalls.cpp
@@ -178,6 +178,14 @@
         return file;
     }
 
+    StatusOr<pid_t> fork() const override {
+        pid_t rv = ::fork();
+        if (rv == -1) {
+            return statusFromErrno(errno, "fork() failed");
+        }
+        return rv;
+    }
+
     StatusOr<int> vfprintf(FILE* file, const char* format, va_list ap) const override {
         auto rv = ::vfprintf(file, format, ap);
         if (rv == -1) {