Add get_fname_rnd for unit tests that want a unique path every time

src/test/test.h

@@ -73,6 +73,7 @@
     {print_ = (I64_PRINTF_TYPE) value_;}, {}, TT_EXIT_TEST_FUNCTION)
 
 const char *get_fname(const char *name);
+const char *get_fname_rnd(const char *name);
 struct crypto_pk_t *pk_generate(int idx);
 
 #define US2_CONCAT_2__(a, b) a ## __ ## b

src/test/testing_common.c

@@ -125,6 +125,18 @@ get_fname(const char *name)
   return get_fname_suffix(name, NULL);
 }
 
+/** Return a filename with a random suffix, relative to our testing temporary
+ * directory. If name is NULL, return the name of the testing temporary
+ * directory, without any suffix. */
+const char *
+get_fname_rnd(const char *name)
+{
+  char rnd[256], rnd32[256];
+  crypto_rand(rnd, RAND_PATH_BYTES);
+  base32_encode(rnd32, sizeof(rnd32), rnd, RAND_PATH_BYTES);
+  return get_fname_suffix(name, rnd32);
+}
+
 /* Remove a directory and all of its subdirectories */
 static void
 rm_rf(const char *dir)