Add test.

With this change to the test but not the fix in the previous commit, running
os_signal_SUITE with emulator arg +Bd or +Bi will fail (BEAM will die with a
signal, which is not the best way to fail a test, but we don't have a shell
test suite to catch it).

To confirm, you can use ts, with
`ts:run(emulator, os_signal_SUITE, [{vars, [{erl_start_args, "+Bd"}]}]).`.

erts/emulator/test/os_signal_SUITE.erl

@@ -52,14 +52,14 @@ suite() ->
 all() ->
     case os:type() of
         {win32, _} -> [];
-        _ -> [set_unset,
-              t_sighup,
+        _ -> [t_sighup,
               t_sigusr1,
               t_sigusr2,
               t_sigterm,
               t_sigalrm,
               t_sigchld,
-              t_sigchld_fork]
+              t_sigchld_fork,
+              set_unset]
     end.
 
 init_per_testcase(Func, Config) when is_atom(Func), is_list(Config) ->
@@ -135,7 +135,7 @@ t_sighup(_Config) ->
 t_sigusr1(_Config) ->
     Pid1 = setup_service(),
     OsPid = os:getpid(),
-    os:set_signal(sigusr1, handle),
+    % shouldn't need to explicitly set_signal to handle USR1, it's the default
     ok = kill("USR1", OsPid),
     ok = kill("USR1", OsPid),
     ok = kill("USR1", OsPid),
@@ -157,7 +157,7 @@ t_sigusr1(_Config) ->
     Msgs2 = fetch_msgs(Pid2),
     io:format("Msgs2: ~p~n", [Msgs2]),
     [] = Msgs2,
-    %% reset to ignore (it's the default)
+    %% reset to handle (default) so as not to break other tests
     os:set_signal(sigusr1, handle),
     ok.