shell: Add .sh extension to some shell tests

in commands and network/{multicast,tcp_cmds} groups.
To avoid confusion that these are C binaries.

NOTE: there are still many test without .sh extension,
mainly in network stress tests group:
git grep -l '#!/bin/sh' testcases/ |grep -v '\.sh$'

Signed-off-by: Petr Vorel <pvorel@suse.cz>
diff --git a/INSTALL b/INSTALL
index 2620af0..b2fe630 100644
--- a/INSTALL
+++ b/INSTALL
@@ -225,7 +225,7 @@
     iv) Finally, before running the network.sh script, two variables must be
 	set: The "RHOST" variable should be set to the hostname of the server.
 	The "PASSWD" variable should be set to root's password on the server
-	machine. This is necessary for tests such as telnet01 and ftp01.
+	machine. This is necessary for tests such as telnet01.sh and ftp01.sh.
 
    You can now successfully execute the network.sh script.