ntp: don't send useless requests in interleaved client mode

In interleaved client mode, when so many consecutive requests were lost
that the first valid (interleaved) response would be dropped for being
too old, switch to basic mode so the response can be accepted if it
doesn't fail in the other tests.

This reworks commit 16afa8eb50.

test/unit/ntp_core.c

@@ -251,7 +251,8 @@ test_unit(void)
     for (j = 0; j < 50; j++) {
       DEBUG_LOG("iteration %d, %d", i, j);
 
-      interleaved = random() % 2;
+      interleaved = random() % 2 && (inst->mode != MODE_CLIENT ||
+                                     inst->tx_count <= MAX_CLIENT_INTERLEAVED_TX);
       authenticated = random() % 2;
       valid = (!interleaved || (source.params.interleaved && has_updated)) &&
               (!source.params.authkey || authenticated);