TAS: fix panic in e2e test when parallelism < completions

[mimowo]

What type of PR is this?

/kind bug /kind flake

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #3620

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mimowo

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[mimowo]

/assign @mbobrovskyi

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[mimowo]

cc @PBundyra

[mimowo]

/test pull-kueue-test-integration-main unrelated, opened issue: https://github.com/kubernetes-sigs/kueue/issues/3633

[mbobrovskyi]

/lgtm Thanks!

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 1668a2c4e6f2289cd61395ae12e61b5dc208a0bb

[mbobrovskyi]

/retest

Due to know issue.