Add test with a lot of different kind of M_LET with list of strings.

m-string.h

@@ -1736,7 +1736,7 @@ string_utf8_p(string_t str)
 #ifndef __cplusplus
 /* Initialize a constant string with the given C string */
 # define STRING_CTE(s)                                                        \
-  ((const string_t){{.u.heap = { .size = sizeof(s)-1, .alloc = sizeof(s) } ,  \
+  (string_srcptr)((const string_t){{.u.heap = { .size = sizeof(s)-1, .alloc = sizeof(s) } , \
         .ptr = ((struct { long long _n; char _d[sizeof (s)]; }){ 0, s })._d }})
 #else
 namespace m_lib {