Add a tricky test to verify disowning an in-flight job

There's some logic in fish to prevent blowing up when an under-construction
job is disowned. Add a test for it.

tests/checks/disown-parent.fish

@@ -0,0 +1,14 @@
+# RUN: env fish_test_helper=%fish_test_helper %fish %s
+
+# Ensure that a job which attempts to disown itself does not explode.
+# Here fish_test_helper is the process group leader; we attempt to disown
+# its pid within a pipeline containing it.
+
+function disowner
+    read -l pid
+    echo Disown $pid
+    disown $pid
+end
+$fish_test_helper print_pid_then_sleep | disowner
+
+# CHECK: Disown {{\d+}}