Add tests for $status after while in various scenarios

Includes a regression test for #5513 and asserts the behavior defined in
\#4982.

tests/while.in

@@ -0,0 +1,27 @@
+# vim: set ft=fish:
+
+function never_runs
+	while false
+	end
+end
+
+function early_return
+	while true
+		return 2
+	end
+end
+
+function runs_once
+	set -l i 1
+	while test $i -ne 0 && set i (math $i - 1)
+	end
+end
+
+# this should return 1
+never_runs; echo $status
+
+# this should return 0
+runs_once; echo $status
+
+# this should return 2
+early_return; echo $status

tests/while.out

@@ -0,0 +1,3 @@
+1
+0
+2