fix(e2e): cap the clusters one suite binary builds at once

go test defaults -parallel to GOMAXPROCS, so peak live clusters follows the
runner's core count: 8 today, from fluentd-aggregator's 4 tests plus volumedrain's
2 plus one each from two more suites. Every one of them imports the same six
images, and all observed 16m deadline hits were kind load, none were kind create.
-parallel 2 caps the peak at 6 and stops it growing with the core count.

This reverses the reasoning that dropped -parallel 1 from kube-logging#2301, which measured
fluentd-aggregator alone. elasticsearch-multiversion has two test functions but
only one builds a cluster, so -parallel cannot serialise the suite; it is instead
the largest beneficiary of a lower peak, because its runtime is mostly
Elasticsearch readiness and that is what starves first. Measured: 856.4s at
-parallel 4 (30625716801) against 387.9s at -parallel 1 (30590989952), while
fluentd-aggregator went 272.5s to 490.7s. The longest package, which is what the
job waits for, therefore fell from 856s to 491s.

The kind command cap stays at 16m. Anchoring it to healthy timings was already
measured as a regression: at 10m, runs failed that would have passed, because a
degraded runner is 2.3-6.2x slower and the loads complete rather than wedge.

-parallel 1 and 4 are measured; 2 is interpolated between them. If load timeouts
survive this, 1 is the fallback and has two green runs behind it.

Signed-off-by: Bence Csati <bence.csati@axoflow.com>

Author: csatib02

Makefile

@@ -53,6 +53,11 @@ OPERATOR_IMG_DEBUG ?= controller:debug
 CRD_OPTIONS ?= crd:maxDescLen=0
 
 E2E_TEST_TIMEOUT ?= 20m
+
+# Clusters one suite binary builds at once. Caps peak concurrency, which
+# otherwise follows the core count and starves the aggregators.
+E2E_SUITE_PARALLEL ?= 2
+
 TEST_COV_DIR := $(shell mkdir -p build/_test_coverage && realpath build/_test_coverage)
 
 CONTROLLER_GEN := ${BIN}/controller-gen
@@ -242,7 +247,7 @@ test-e2e-nodeps:
 		KIND_IMAGE="$(KIND_IMAGE)" \
 		PROJECT_DIR="$(PWD)" \
 		E2E_TEST_COV_DIR=${TEST_COV_DIR} \
-		go test -count=1 -v -timeout ${E2E_TEST_TIMEOUT} $$(go list ./${E2E_TEST}/... | grep -vE '/e2e/(common|internal)(/|$$)')
+		go test -count=1 -v -parallel ${E2E_SUITE_PARALLEL} -timeout ${E2E_TEST_TIMEOUT} $$(go list ./${E2E_TEST}/... | grep -vE '/e2e/(common|internal)(/|$$)')
 		go tool covdata textfmt -i=${TEST_COV_DIR}/covdatafiles -o ${TEST_COV_DIR}/coverage_e2e.out
 	@echo "--- E2E test coverage report"
 	go tool covdata percent -i=${TEST_COV_DIR}/covdatafiles