fix(libsinsp/test): fix async_key_value_source flakiness #1825

Job	Run time
build-test-e2e-amd64 😇 (bundled_deps)	13m 35s
build-test-e2e-arm64 😇 (bundled_deps)	25m 24s
test-e2e-amd64-kmod 😇 (bundled_deps)	9m 4s
test-e2e-amd64-bpf 😇 (bundled_deps)	11m 3s
test-e2e-amd64-modern-bpf 😇 (bundled_deps)	11m 6s
test-e2e-arm64-kmod 😇 (bundled_deps)	14m 16s
test-e2e-arm64-bpf 😇 (bundled_deps)	6h 0m 12s
test-e2e-arm64-modern-bpf 😇 (bundled_deps)	21m 1s
	7h 45m 41s

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