Run all exim string expansion tests with empty config file.

Added a short note on memory management.

src/libexim-encrypt-dlfunc-decrypt-secretbox.c

@@ -7,6 +7,10 @@
 #include <stdbool.h>
 #include "common.c"
 
+/* A note on memory management: this code lacks calls to free() for every malloc()ed piece of memory. This is deliberate
+ * as these processes are short-lived and calling free() right before an exit() seems kind of moot.
+ */
+
 #define ENVVAR_PASSWORD_NAME "LIBEXIM_PASSWORD"
 
 void print_usage(char *progname) {