test-mbed_drivers-ticker: fix ticker cross attach

This commit fixes ticker cross-schedule bug in test_case_2x_callbacks subtest

In effect of this bug:
    ticker_callback_1_switch_to_2 was called only once
    ticker2 was never been fired because it was repeatedly detached just before fire and attached again

Author: maciejbocianski

TESTS/mbed_drivers/ticker/main.cpp

@@ -69,7 +69,9 @@ void ticker_callback_1_switch_to_2(void)
     // If ticker is NULL then it is being or has been deleted
     if (ticker1) {
         ticker1->detach();
-        ticker1->attach_us(ticker_callback_2_switch_to_1, ONE_MILLI_SEC);
+    }
+    if (ticker2) {
+        ticker2->attach_us(ticker_callback_2_switch_to_1, ONE_MILLI_SEC);
     }
     switch_led1_state();
 }
@@ -80,7 +82,9 @@ void ticker_callback_2_switch_to_1(void)
     // If ticker is NULL then it is being or has been deleted
     if (ticker2) {
         ticker2->detach();
-        ticker2->attach_us(ticker_callback_1_switch_to_2, ONE_MILLI_SEC);
+    }
+    if (ticker1) {
+        ticker1->attach_us(ticker_callback_1_switch_to_2, ONE_MILLI_SEC);
     }
     switch_led2_state();
 }