Move test_password_{one,many} to libcrypt-util.c

They are only used under src/home/, but I want to add tests in test-libcrypt-util.c.
And the functions are almost trivial, so I think it is OK to move them to shared.

src/shared/libcrypt-util.h

@@ -23,3 +23,5 @@ static inline int hash_password(const char *password, char **ret) {
         return hash_password_full(password, NULL, NULL, ret);
 }
 bool looks_like_hashed_password(const char *s);
+int test_password_one(const char *hashed_password, const char *password);
+int test_password_many(char **hashed_password, const char *password);