[chore] [exporter/splunkhecexporter] make large_batch test less demanding (#16322)
[chore] [exporter/splunkhecexporter] make large_batch test less demanding (#16322)

Make flaky test less demanding, and add more checks to make sure the test actually tests a behavior of the exporter
open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib