add a stresstest tool for rapid connects and disconnects for iiod

This is based on an iiod robustness test that Lars did:
dc33705

This cleans things up, adds a few more options.

On 2018R1 release (ZedBoard + FMCOMMS3) would cause kernel panics (Scheduling
while atomic) in a few hours over ethernet.

On 2019 release (same hardware), can run for 3 days, doing 45 context
open/destroy per second. (over 11M in total, & 128k where the open/close
was simultaneous in different threads).

Output looks like:
total: 2d21h31m21s Context : 11510707 (45.99/s), buffers: 57504276 (229.76/s), refills : 287540521 (1148.87/s)
    0        :  128395 ( 1.12%)
  1 - 9   μs :  338154 ( 2.94%)
 10 - 99  μs :  466087 ( 4.05%)
100 - 999 μs : 1155429 (10.04%)
  1 - 9.9 ms : 2774553 (24.10%)
 10 - 99  ms : 6521903 (56.66%)
100 - 999 ms :  125908 ( 1.09%)
over 1 s     :       0 ( 0.00%)

Signed-off-by: Robin Getz <[email protected]>

tests/CMakeLists.txt

@@ -40,9 +40,12 @@ set(IIO_TESTS_TARGETS iio_genxml iio_info iio_attr iio_readdev iio_reg iio_write
 
 if(PTHREAD_LIBRARIES)
 	project(iio_adi_xflow_check C)
+	project(iio_stresstest C)
 	add_executable(iio_adi_xflow_check iio_adi_xflow_check.c)
+	add_executable(iio_stresstest iio_stresstest.c)
 	target_link_libraries(iio_adi_xflow_check iio ${PTHREAD_LIBRARIES})
-	set(IIO_TESTS_TARGETS ${IIO_TESTS_TARGETS} iio_adi_xflow_check)
+	target_link_libraries(iio_stresstest iio ${PTHREAD_LIBRARIES})
+	set(IIO_TESTS_TARGETS ${IIO_TESTS_TARGETS} iio_adi_xflow_check iio_stresstest)
 
 	target_link_libraries(iio_readdev  ${PTHREAD_LIBRARIES})
 	target_link_libraries(iio_writedev ${PTHREAD_LIBRARIES})