test: Add hs_dos.c unit tests

Currently test the only available function which is hs_dos_can_send_intro2()
within the HS anti-DoS subsystem.

Closes #15516

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/test/test.c

@@ -877,6 +877,7 @@ struct testgroup_t testgroups[] = {
   { "hs_config/", hs_config_tests },
   { "hs_control/", hs_control_tests },
   { "hs_descriptor/", hs_descriptor },
+  { "hs_dos/", hs_dos_tests },
   { "hs_intropoint/", hs_intropoint_tests },
   { "hs_ntor/", hs_ntor_tests },
   { "hs_service/", hs_service_tests },