test: fix flaky interval test

PR-URL: #18161
Fixes: #18160
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: apapirovski

test/sequential/test-timers-set-interval-excludes-callback-duration.js

@@ -8,10 +8,10 @@ let first;
 const t = setInterval(() => {
   cntr++;
   if (cntr === 1) {
-    first = Timer.now();
     common.busyLoop(100);
+    first = Timer.now();
   } else if (cntr === 2) {
-    assert(Timer.now() - first < 120);
+    assert(Timer.now() - first < 100);
     clearInterval(t);
   }
 }, 100);