test(devshard): de-flake TestGRPCProvider_ServerNotSyncedPausesBetweenPolls

[ouicate]

Problem

TestGRPCProvider_ServerNotSyncedPausesBetweenPolls slept a fixed 30ms after
New() and then asserted that exactly one call had been made. But New() starts
the long-poll loop asynchronously (go newGRPCRunner(...).run(...)), so on a
loaded runner the goroutine's first gRPC call may not have landed within 30ms —
the test then fails with expected 1, got 0. This is a timing flake unrelated to
the code under test (it surfaced on an unrelated PR's CI: the "Build and Test
Devshard" job for #1410).

Fix

Replace the fixed-delay assumptions with require.Eventually waits:

• wait for the initial fetch (tolerant of async goroutine startup), then assert
  exactly one call so far — the next poll is one ErrorBackoffMin (50ms) away, so
  a single call still proves the loop is pacing itself, not busy-looping;
• wait for the second poll after the backoff, then bound the call count across a
  short window to preserve the original "paced, not a busy loop" guarantee.

No production code changes.

Tests

go test ./runtimeconfig/ -run TestGRPCProvider_ServerNotSyncedPausesBetweenPolls -count=100
and -count=60 -cpu=1 are both green; go vet and gofmt are clean.