tests: Remove grep process entries from pidgrep

Problem:
We were picking process with lowest pid from ps|grep
result. However, lowest pid need not be oldest process
as recycling of PIDs can take place.

Solution:
Removed grep process entries from ps entries using 
grep -v grep.

Change-Id: I2b9687a05a34cf6358f773183770d69a3fb9eb10
BUG: 858488
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/4765
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>

tests/bugs/bug-858488-min-free-disk.t

@@ -7,7 +7,7 @@ cleanup;
 
 function pidgrep()
 {
-    ps ax | grep "$1" | awk '{print $1}' | head -1
+    ps ax | grep "$1" | grep -v grep | awk '{print $1}' | head -1
 }
 
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