[linux-lvm] It looks wrong for the timeout when lvm test running

[Heming Zhao <heming.zhao@suse.com>]

Hello List,

The lvm test default timeout (--timeout) default value is 180.
But I met below condition when running testcase (in virtual machine):
```
make check_local T=shell/snapshot-merge.sh
    ... ...
###       passed: [ndev-vanilla] shell/snapshot-merge.sh 665
    ... ...
```

Obviously, the timeout doesn't work normally.
If I change below code, the timeout work as expected. (in lib/brick-shelltest.h)
```
         if ( select( nfds, &set, NULL, NULL, &wait ) > 0 ) {
             silent_start = end; /* something happened */
             io.sync( false );
         }
```

And after change the code, some testcase will become timeout, which may pass before.


The whole patch as below:
```
 From bdbd569c2b9ab49d4a4246f203b1cd9f7049db97 Mon Sep 17 00:00:00 2001
From: Zhao Heming <heming.zhao@suse.com>
Date: Mon, 9 Dec 2019 17:09:17 +0800
Subject: [PATCH] test: fix timeout abnormal issue

---
  test/lib/brick-shelltest.h | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/lib/brick-shelltest.h b/test/lib/brick-shelltest.h
index b58f3d6843..12e08249f3 100644
--- a/test/lib/brick-shelltest.h
+++ b/test/lib/brick-shelltest.h
@@ -822,7 +822,7 @@ struct TestCase {
              }
          }
          if ( select( nfds, &set, NULL, NULL, &wait ) > 0 ) {
-            silent_start = end; /* something happened */
+            //silent_start = end; /* something happened */
              io.sync( false );
          }
  
-- 
2.24.0
```

Thanks.