stream: halve the run time of re-subscribe test

Even given the comment, 1s might be excessively large.

Author: dynst

packages/stream/src/concat.spec.ts

@@ -214,7 +214,7 @@ describe("concat", () => {
     });
     // For browsers and CI, clocks and runtimes are very unreliable.
     // Especially Mac+Firefox on Travis makes big trouble. Thus we need huge intervals.
-    const intervalDuration = 1000;
+    const intervalDuration = 500;
     const producerActiveLog = new Array<boolean>();
 
     let producerInterval: NodeJS.Timeout;