fix(test): wait for microVM to boot before snapshotting

In test_vulnerabilities.py we have various tests that check whether a
condition holds after resuming from a snapshot. These checks seem to
consistently fail if we take a snapshot before letting the guest kernel
boot.

Introduce an ssh command to ensure that the guest has booted before
taking the snapshot so that we avoid the issue.

Signed-off-by: Babis Chalios <bchalios@amazon.es>

tests/integration_tests/security/test_vulnerabilities.py

@@ -122,6 +122,9 @@ def with_restore(factory, microvm_factory):
 
     def restore(firecracker=None, jailer=None):
         microvm = factory(firecracker, jailer)
+        # Ensure that we have booted before getting the snapshot.
+        rc, _, stderr = microvm.ssh.run("true")
+        assert rc == 0, stderr
         snapshot = microvm.snapshot_full()
 
         if firecracker: