tests/monocypher: Increase stack space

As should have been done in the initial commit. Stack space set to the
amount described in the doc

tests/pkg_monocypher/Makefile

@@ -5,6 +5,11 @@ BOARD_BLACKLIST := arduino-duemilanove arduino-mega2560 arduino-uno chronos \
         jiminy-mega256rfr2 mega-xplained msb-430 msb-430h telosb \
         waspmote-pro wsn430-v1_3b wsn430-v1_4 z1
 
+BOARD_INSUFFICIENT_MEMORY := nucleo-f031k6 nucleo-f042k6
+
+# required for Monocypher (as described in the package documentation)
+CFLAGS += "-DTHREAD_STACKSIZE_MAIN=(3072 + THREAD_STACKSIZE_DEFAULT + THREAD_EXTRA_STACKSIZE_PRINTF)"
+
 USEMODULE += embunit
 USEMODULE += random
 USEPKG += monocypher