Tweak tests, consolidate pins target validation (#21254)

Marlin/src/pins/mega/pins_MIGHTYBOARD_REVE.h

@@ -37,9 +37,8 @@
  *  number (B5) agrees with the schematic but B5 is assigned to logical pin 11.
  */
 
-#if NOT_TARGET(__AVR_ATmega1280__, __AVR_ATmega2560__)
-  #error "Oops! Select 'Mega 1280' or 'Mega 2560' in 'Tools > Board.'"
-#endif
+#define ALLOW_MEGA1280
+#include "env_validate.h"
 
 #define BOARD_INFO_NAME      "Mightyboard"
 #define DEFAULT_MACHINE_NAME "MB Replicator"