Use fork()/exec() in unit test code

This makes the unit test code more like the actual code used by
end-users, and therefore makes the tests more accurate.

This trips a bug in the code which will be fixed later, requiring a test
to be changed to compensate.

Author: DemiMarie

agent/qrexec-agent.c

@@ -230,16 +230,20 @@ _Noreturn void do_exec(const char *prog, const char *cmd, const char *user)
                 user, pw->pw_name);
             exit(QREXEC_EXIT_PROBLEM);
         }
-        /* call QUBESRPC if requested */
-        if (prog) {
-            /* no point in creating a login shell for test environments */
-            exec_qubes_rpc2(prog, cmd, environ, false);
-        }
 
-        /* otherwise exec shell */
-        execl("/bin/sh", "sh", "-c", cmd, NULL);
-        PERROR("execl");
-        exit(QREXEC_EXIT_PROBLEM);
+        /* FORK HERE */
+        child = fork();
+
+        switch (child) {
+            case -1:
+                goto error;
+            case 0:
+                really_exec(prog, pw, environ, cmd);
+            default:
+                /* parent */
+                close_std();
+                exit(really_wait(child));
+        }
     }
 
     pw = getpwnam(user);

qrexec/tests/socket/agent.py

@@ -602,7 +602,7 @@ def test_exec_service_bad_service(self):
             messages[-2:],
             [
                 (qrexec.MSG_DATA_STDERR, b""),
-                (qrexec.MSG_DATA_EXIT_CODE, b"\175\0\0\0"),
+                (qrexec.MSG_DATA_EXIT_CODE, b"\1\0\0\0"),
             ],
         )
         for msg_type, msg_value in messages[1:-2]: