fix: flaky TestPrunePromptTermination test

[hirehamir]

- What I did

Addressed #5802

- How I did it

I kept the write end of the pipe alive for the duration of the test with

defer w.Close()

- How to verify it

# without the changes from this branch: fails most runs
# with the changes from this branch: 0 failures
GOGC=1 GOMEMLIMIT=8MiB go test ./cli/command/container/ -run TestContainerPrunePromptTermination -count=40 -v

	GC forced (GOGC=1 GOMEMLIMIT=8MiB)	GC off (GOGC=off)
without this change	33 of 40 fail	0 of 40 fail
with this change	0 of 40 fail	0 of 40 fail

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/test/cmd.go	0.00%	2 Missing ⚠️

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