Removes existing certificates when run mkcert install.

To avoid generating a wrong certificate chain (once previous mkcert generated certificates exist).

Bug: 214337220
Test: acloud create
Change-Id: I0e058c3612fe77c36d6a3faec9b2fbfbd5facb08
diff --git a/setup/host_setup_runner_test.py b/setup/host_setup_runner_test.py
index c233094..d98466b 100644
--- a/setup/host_setup_runner_test.py
+++ b/setup/host_setup_runner_test.py
@@ -205,6 +205,7 @@
         self.Patch(mkcert, "Install")
         self.LocalCAHostSetup.Run()
         mkcert.Install.assert_called_once()
+        mkcert.Install.reset_mock()
 
         self.Patch(LocalCAHostSetup, "ShouldRun", return_value=False)
         self.LocalCAHostSetup.Run()
@@ -213,7 +214,7 @@
         self.Patch(utils, "GetUserAnswerYes", return_value=False)
         self.Patch(sys, "exit")
         self.LocalCAHostSetup.Run()
-        sys.exit.assert_called_once()
+        mkcert.Install.assert_not_called()
 
 
 if __name__ == "__main__":