[chore] Fix Makefile's INTEGRATION_MODS

[djaglowski]

This properly deduplicates integration test modules.

Can be manually validated by adding a simple make target:

int-modules:
	@echo $(INTEGRATION_MODS) | tr ' ' '\n' | sort

./exporter/azuredataexplorerexporter
./extension/observer/dockerobserver
./pkg/stanza/adapter
./processor/k8sattributesprocessor
./receiver/activedirectorydsreceiver
./receiver/aerospikereceiver
./receiver/apachereceiver
./receiver/awscloudwatchreceiver
./receiver/bigipreceiver
./receiver/cloudflarereceiver
./receiver/dockerstatsreceiver
./receiver/elasticsearchreceiver
./receiver/flinkmetricsreceiver
./receiver/iisreceiver
./receiver/jmxreceiver
./receiver/jmxreceiver/internal/subprocess
./receiver/kafkametricsreceiver
./receiver/memcachedreceiver
./receiver/mongodbatlasreceiver
- ./receiver/mongodbatlasreceiver
./receiver/mongodbreceiver
./receiver/mysqlreceiver
./receiver/nginxreceiver
./receiver/postgresqlreceiver
./receiver/rabbitmqreceiver
./receiver/redisreceiver
./receiver/riakreceiver
./receiver/snmpreceiver
./receiver/sqlqueryreceiver
./receiver/vcenterreceiver
./receiver/zookeeperreceiver