tests: Default integration test threads to 1

[Jakob-Naucke]

Let test executors decide how many threads they want on the (very memory-intensive) integration tests. Some CI runners, such as openshift-ci, do not have the resources to run 3 at a time.

@alicefr or, if you prefer, something like

# If $THREADS is set, multi-thread integration tests with $THREADS but at most 3.
# Otherwise, do not multi-thread integration tests.
THREADS ?= 1
INTEGRATION_TEST_THREADS = $$(( $(THREADS) > 3 ? 3 : $(THREADS) ))

@yalzhang idk if we want to just merge this or you want to patch it into openshift/release#73211 for testing (but then we should remove it before merging)

[openshift-ci]

@Jakob-Naucke: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/operator-lifecycle-verify	4dca750	link	true	/test operator-lifecycle-verify
ci/prow/infra-provision-verify	4dca750	link	true	/test infra-provision-verify

Full PR test history. Your PR dashboard.

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[Jakob-Naucke]

The test over at openshift/release#73211 passed with this patch applied. @yalzhang lmk what a smart order to merge the PRs would be.

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: alicefr, Jakob-Naucke

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[alicefr]

This looks great!

[alicefr]

Maybe we can enable and use KSM to run multiple VMs in parallel for the future. It might help reducing the memory footprint of the CI