Test return builtin doesn't map negative numbers to zero

Prior to 1811a2d, the return value for negative return codes was UB and I'd
witnessed both expected cases like -256 mapping to a $status of 0 and unexpected
cases like a return value of -1 mapping to a $status of 0. As such, this doesn't
test just one fixed return value but the entire range from negative multiples of
256 all the way down (rather, up!) to -1.

tests/checks/return.fish

@@ -20,3 +20,16 @@ $fish -ic 'echo interactive-foo; return 69; echo interactive-bar'
 # but not bar
 echo $status
 # CHECK: 69
+
+# Verify negative return values don't cause UB and never map to 0
+function empty_return
+    return $argv[1]
+end
+
+for i in (seq -550 -1)
+    empty_return $i
+    if test $status -eq 0
+        echo returning $i from a fish script maps to a $status of 0!
+    end
+end
+# CHECK: