scrape_test: add scrape offset in test scraper to avoid race

This change adds an offset to the test scraper so a scrape is not
initiated when the context is cancelled. Without this change, there
was a race between the context cancellation and the scrape jitter wait
and this change fixes that.

Can verify using:
go test -run ^TestScrapeLoopStopBeforeRun$ github.com/prometheus/prometheus/scrape -race -count 1000

Signed-off-by: Ridwan Sharif <ridwanmsharif@google.com>

scrape/scrape_test.go

@@ -647,6 +647,10 @@ func TestScrapeLoopStopBeforeRun(t *testing.T) {
 		nil,
 	)
 
+	// The loop must terminate during the initial offset if the context
+	// is canceled.
+	scraper.offsetDur = time.Hour
+
 	// The scrape pool synchronizes on stopping scrape loops. However, new scrape
 	// loops are started asynchronously. Thus it's possible, that a loop is stopped
 	// again before having started properly.