test(engine): align goroutine leak tolerance with the rest of the suite

TestNoGoroutineLeakOnPrematureCancel used "leak > 2" while every other
leak check in the codebase allows 3: scan_test.go:206 (before+3) in the same
file, and scanner_test.go:101 (leak > 3). This makes it consistent.

Rebased from #147 onto current develop; the other changes in that PR had
already landed via #145.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: mendsec

pkg/engine/scan_test.go

@@ -320,7 +320,7 @@ func TestNoGoroutineLeakOnPrematureCancel(t *testing.T) {
 
 	after := runtime.NumGoroutine()
 	leak := after - before
-	if leak > 2 { // tolerance for test harness goroutines
+	if leak > 3 { // tolerance for test harness goroutines
 		t.Errorf("goroutine leak detected: %d goroutines created and not cleaned up", leak)
 	}
 }