Regression test for destroy cell queue

This exercises the code that crashed and caused #9296.

src/test/include.am

@@ -20,6 +20,7 @@ src_test_test_SOURCES = \
 	src/test/test_addr.c \
 	src/test/test_cell_formats.c \
 	src/test/test_circuitlist.c \
+	src/test/test_circuitmux.c \
 	src/test/test_containers.c \
 	src/test/test_crypto.c \
 	src/test/test_cell_queue.c \

src/test/test.c

@@ -2133,6 +2133,7 @@ extern struct testcase_t introduce_tests[];
 extern struct testcase_t replaycache_tests[];
 extern struct testcase_t cell_format_tests[];
 extern struct testcase_t circuitlist_tests[];
+extern struct testcase_t circuitmux_tests[];
 extern struct testcase_t cell_queue_tests[];
 extern struct testcase_t options_tests[];
 
@@ -2152,6 +2153,7 @@ static struct testgroup_t testgroups[] = {
   { "replaycache/", replaycache_tests },
   { "introduce/", introduce_tests },
   { "circuitlist/", circuitlist_tests },
+  { "circuitmux/", circuitmux_tests },
   { "options/", options_tests },
   END_OF_GROUPS
 };