Test --wrapper when restarting process.

Test --wrapper when restarting process.

My patch series will affect the code starting inferior in GDBserver
(callees of start_inferior), so we need tests to cover how
start_inferior is used in different cases.

In server.c:process_serial_event, start_inferior is used when
GBDserver receives 'R' packet, and this patch is to add a test
for this path, and see how --wrapper option works when the process
is restarted.

gdb/testsuite:

2015-07-24  Yao Qi  <yao.qi@linaro.org>

	* gdb.server/ext-wrapper.exp: Test --wrapper option when
	restarting process.