Fix some sched and tty tests

The ps sched test has been disabled. There are too many
odd build farms this fails in strange ways.

Other odd build farms have no tty and so some tests check
for no tty and skip if not found.

testsuite/config/unix.exp

@@ -146,3 +146,12 @@ proc kill_testproc { } {
     kill_process $testproc2_pid
     file delete $testproc_path
 }
+
+proc get_tty {} {
+    if { [catch { set raw_tty [ exec tty ] } msg]} {
+        warning "No TTY found"
+        return ""
+    }
+    regexp "/dev/(.+)" $raw_tty > tty
+    return $tty
+}