Fixing dev_null test

The dev null test was returning failure from the device even though the
device can't determine the success of its own test. Therefore I've removed
this line. The device now lets the host test determines the success of the
test.

TESTS/mbed_drivers/dev_null/main.cpp

@@ -44,5 +44,8 @@ int main() {
         printf("{{to_null;printf redirected to /null}}\n");
         printf("MBED: this printf is already redirected to /null\n");
     }
-    GREENTEA_TESTSUITE_RESULT(false);
+
+    while(1) {
+      // Success is determined by the host test at this point, so busy wait
+    }
 }