fix(xcat-core): bound the pings that assert a purged node stopped resolving

nodepurge/cases0 asserted that testnode1 and testnode2 no longer resolve by running 'ping'
with no count and expecting a non-zero exit. When the name does not resolve the ping fails
immediately, which is the passing path -- but when it DOES resolve, which is the regression
the case exists to catch, the ping never returns. The cell stops there and is killed by the
pipeline timeout, taking the whole run's JUnit with it, so the one case that finds a real
defect is also the one that hides every other result.

Bound both with -c 1 -w 2. The assertion is unchanged: a name that does not resolve, or
resolves to something that does not answer, still exits non-zero.

autotest_ping_bounded.t goes green on this commit.

Closes #59.

Signed-off-by: Daniel Hilst <392820+dhilst@users.noreply.github.com>
(cherry picked from commit 187daabe3601dfe0448656c446994d91d8f794aa)

xCAT-test/autotest/testcase/nodepurge/cases0

@@ -21,9 +21,9 @@ cmd:ls /install/autoinst/testnode1*
 check:output=~No such file or directory
 cmd:ls /install/autoinst/testnode2*
 check:output=~No such file or directory
-cmd:ping testnode1
+cmd:ping -c 1 -w 2 testnode1
 check:rc!=0
-cmd:ping testnode2
+cmd:ping -c 1 -w 2 testnode2
 check:rc!=0
 end