doctest_fix: try setting fixed default terminal size #16804

Job	Run time
doctest (1.10, ubuntu-latest)	25m 58s
doctest (1.11-nightly, ubuntu-latest)	28m 40s
doctest (nightly, ubuntu-latest)	25m 35s
test (1.6, short, ubuntu-latest)	1h 30m 36s
doctest (1.10, ubuntu-latest, depwarn=error)	26m 15s
test (1.6, long, ubuntu-latest)	58m 8s
doctest (1.10, macOS, RPTU)	12m 23s
test (1.10, short, ubuntu-latest)	1h 13m 30s
test (1.10, long, ubuntu-latest)	49m 31s
test (1.11-nightly, short, ubuntu-latest)	1h 24m 37s
test (1.11-nightly, long, ubuntu-latest)	54m 32s
test (nightly, short, ubuntu-latest)	1h 22m 4s
test (nightly, long, ubuntu-latest)	51m 49s
test (1.10, short, ubuntu-latest, depwarn=error)	1h 12m 27s
test (1.10, long, ubuntu-latest, depwarn=error)	44m 31s
test (1.10, short, macOS, RPTU)	13m 51s
test (1.10, long, macOS, RPTU)	14m 22s
test (1.10, book, ubuntu-latest)	55m 0s
	14h 23m 49s

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