tests/tap_runner: intercept stderr

[igaw]

Obviously, besides intercepting stdout, the tap runner should also intercept stderr.

root@79d7b51a2cc1:/build/nvme-cli# /usr/bin/python3 /build/nvme-cli/tests/tap_runner.py --start-dir /build/nvme-cli/tests nvme_attach_detach_ns_test
TAP version 13
1..1
#
# Using nvme binary 'nvme'
# find: ‘/sys/devices/system/cpu/cpu7/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu5/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu3/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu1/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu6/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu4/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu2/thermal_throttle’: Permission denied
# find: ‘/sys/devices/system/cpu/cpu0/thermal_throttle’: Permission denied
# find: ‘/sys/devices/virtual/powercap’: Permission denied
# /bin/sh: 1: nvme: not found
not ok 1 - test_attach_detach_ns (TestNVMeAttachDetachNSCmd)
# Traceback (most recent call last):
#   File "/usr/lib/python3.13/unittest/case.py", line 58, in testPartExecutor
#     yield
#   File "/usr/lib/python3.13/unittest/case.py", line 647, in run
#     self._callSetUp()
#     ~~~~~~~~~~~~~~~^^
#   File "/usr/lib/python3.13/unittest/case.py", line 603, in _callSetUp
#     self.setUp()
#     ~~~~~~~~~~^^
#   File "/build/nvme-cli/tests/nvme_attach_detach_ns_test.py", line 50, in setUp
#     super().setUp()
#     ~~~~~~~~~~~~~^^
#   File "/build/nvme-cli/tests/nvme_test.py", line 85, in setUp
#     self.ns_mgmt_supported = self.get_ns_mgmt_support()
#                              ~~~~~~~~~~~~~~~~~~~~~~~~^^
#   File "/build/nvme-cli/tests/nvme_test.py", line 237, in get_ns_mgmt_support
#     oacs = to_decimal(self.get_id_ctrl_field_value("oacs"))
#                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
#   File "/build/nvme-cli/tests/nvme_test.py", line 345, in get_id_ctrl_field_value
#     self.assertEqual(err, 0, "ERROR : reading id-ctrl failed")
#     ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# AssertionError: 127 != 0 : ERROR : reading id-ctrl failed
#
#