Fix the flaky tty_ownership test on Mac

The tty_ownership test was sometimes failing. In this test,
`fish_test_helper` creates a child and transfers the tty to it,
"abandoning" the tty. In some cases, the child was running before the
parent; the child claims the tty. When the parent tries to transfer it to
the child, it get SIGTTIN and stops. Fix this by ignoring SIGTTIN and
SIGTTOU.

This only affects macOS and BSDs.

src/fish_test_helper.cpp

@@ -13,6 +13,10 @@
 #include <iterator>  // for std::begin/end
 
 static void abandon_tty() {
+    // The parent may get SIGSTOPed when it tries to call tcsetpgrp if the child has already done
+    // it. Prevent this by ignoring signals.
+    signal(SIGTTIN, SIG_IGN);
+    signal(SIGTTOU, SIG_IGN);
     pid_t pid = fork();
     if (pid < 0) {
         perror("fork");