Add tests for errors during SSL or GSSAPI handshake

These test that libpq correctly falls back to a plaintext connection
on handshake error, in the "prefer" modes.

Reviewed-by: Michael Paquier
Discussion: https://www.postgresql.org/message-id/CAOYmi%2Bnwvu21mJ4DYKUa98HdfM_KZJi7B1MhyXtnsyOO-PB6Ww%40mail.gmail.com

src/backend/libpq/be-secure-gssapi.c

@@ -21,6 +21,7 @@
 #include "libpq/pqformat.h"
 #include "miscadmin.h"
 #include "pgstat.h"
+#include "utils/injection_point.h"
 #include "utils/memutils.h"
 
 
@@ -499,6 +500,8 @@ secure_open_gssapi(Port *port)
 				minor;
 	gss_cred_id_t delegated_creds;
 
+	INJECTION_POINT("backend-gssapi-startup");
+
 	/*
 	 * Allocate subsidiary Port data for GSSAPI operations.
 	 */