Fix longevity test runs (nginx#2065)

Problem:
Running longevity test mistakenly runs all other NFRs
make start-longevity-test and make stop-longevity-test mistakenly
executes all NFRs. Only longevity test must run.

Solution:
Problem was introduced in 18671fb
Ensure the env variables are exported so that they are passed to nfr-test target.

Testing:
- Ran make start-longevity-test and make stop-longevity-test successfully.

CLOSES - nginx#2064

Author: miledxz

tests/Makefile

@@ -100,11 +100,11 @@ nfr-test: ## Run the NFR tests on a GCP VM
 	NFR=true bash scripts/run-tests-gcp-vm.sh
 
 .PHONY: start-longevity-test
-start-longevity-test: START_LONGEVITY=true
+start-longevity-test: export START_LONGEVITY=true
 start-longevity-test: nfr-test ## Start the longevity test to run for 4 days in GKE
 
 .PHONY: stop-longevity-test
-stop-longevity-test: STOP_LONGEVITY=true
+stop-longevity-test: export STOP_LONGEVITY=true
 stop-longevity-test: nfr-test ## Stop the longevity test and collects results
 
 .PHONY: .vm-nfr-test