test

Add assertions for stderr in capture output tests #6070

Job	Run time
package	26s
build (windows-py39-unittestextras)	21m 15s
build (windows-py39-pluggy)	6m 52s
build (windows-py39-xdist)	6m 46s
build (windows-py310)	7m 21s
build (windows-py311)	28m 50s
build (windows-py312)	21m 54s
build (windows-py313)	22m 17s
build (ubuntu-py39-lsof-numpy-pexpect)	26m 15s
build (ubuntu-py39-pluggy)	5m 19s
build (ubuntu-py39-freeze)	45s
build (ubuntu-py39-xdist)	5m 29s
build (ubuntu-py310-xdist)	6m 15s
build (ubuntu-py311)	18m 12s
build (ubuntu-py312)	26m 15s
build (ubuntu-py313-pexpect)	22m 18s
build (ubuntu-pypy3-xdist)	8m 41s
build (macos-py39)	6m 13s
build (macos-py310)	4m 55s
build (macos-py312)	4m 48s
build (macos-py313)	5m 3s
build (doctesting)	51s
build (plugins)	51s
check	4s
	4h 17m 55s

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