test(grpc-js): stabilize weighted round robin tests on slow runners

[olavloite]

In test-weighted-round-robin, test traffic was issued immediately against freshly
created clients with aggressive 100ms per-call deadlines. On slower or heavily
loaded CI environments (such as Windows Kokoro runners), cold-start connection
establishment, name resolution, and picker readiness frequently exceeded 100ms,
triggering intermittent DEADLINE_EXCEEDED ("Waiting for LB pick") failures.
Additionally, blind initial traffic could reach only the first backend before
the second finished connecting, leaving the second backend's metrics unrecorded.
Furthermore, executing 50 sequential RPC round-trips combined with arbitrary
sleep timers pushed total execution time beyond Mocha's default 2-second timeout
on Windows VMs (often failing at ~2.005s).

This change stabilizes the tests by:

1. Ensuring the channel reaches readiness and that both backends have responded
   to initial traffic before measurement begins, guaranteeing that connections
   and initial metric tracking are established across all endpoints.
2. Synchronizing deterministically against picker updates rather than relying on
   fixed sleep delays, and executing measurement RPCs concurrently over HTTP/2
   so the suite runs in ~1s while preserving the expected weight distribution.
3. Increasing the test timeout to provide sufficient headroom against VM
   scheduling jitter on Windows CI.