tests: Add header stdlib.h to allow scan-build to do analysis

Without the header being explicitly included, scan-build run on CI
was returning errors for these files, such as:

include the header <stdlib.h> or explicitly provide a declaration
for 'calloc'

The functions in question were calloc, free, etc.

Change-Id: I4b79c5f86c074c456533296c309293e4064abe3f
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/84455
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

tests/helpers.c

@@ -19,6 +19,7 @@
 #include "flash.h"
 
 #include <stdint.h>
+#include <stdlib.h>
 
 void address_to_bits_test_success(void **state)
 {