[action] [PR:1766] [test] Fix flaky FlexCounter.bulkChunksize by replacing usleep with poll-wait

[mssonicbld]

Description

Fix flaky FlexCounter.bulkChunksize unit test that intermittently fails in CI due to timing-dependent usleep(1000*1050).

Issue: #1765

Root Cause

The test uses usleep(1000*1050) (1.05s) to wait for the FlexCounter polling thread (1s poll interval) to complete counter collection. Under CI load, the polling thread may not finish within this window, causing assertion failures like:

TestFlexCounter.cpp:1390: Failure
Expected equality of these values:
  object_count
    Which is: 6
  unifiedBulkChunkSize
    Which is: 3

This failure reproduces on completely unrelated PRs (#1763, #1764), confirming it is timing-dependent and not caused by code changes.

Fix

Replace all usleep(1000*1050) calls with deterministic poll-wait helpers:

• waitForCounterKeys(table, expectedCount) — polls until the expected number of counter keys appear in COUNTERS_DB
• waitForCounterValue(table, key, field) — polls until a specific counter field has a non-empty value

Both helpers poll every 100ms with a 5-second timeout. This eliminates timing sensitivity while keeping tests fast on unloaded machines (typically completes in 1-2 polls).

Also replaces the similar usleep(1000*1000) and usleep(1000*2000) in queryCounterCapability / addRemoveCounterForPort which have the same timing vulnerability.

Changes

• unittest/syncd/TestFlexCounter.cpp:
  • Add #include <chrono> for steady_clock
  • Add waitForCounterKeys() helper
  • Add waitForCounterValue() helper
  • Replace 8x usleep(1000*1050), 1x usleep(1000*1000), 1x usleep(1000*2000) across testAddRemoveCounter, FlexCounter.counterIdChange, FlexCounter.addRemoveCounterForPort, and testDashMeterAddRemoveCounter
  • Retain usleep(60*1000) in updateSwitchDebugCounterIdList (50ms poll interval, not flaky)

Signed-off-by: Sonic Build Admin sonicbld@microsoft.com

[mssonicbld]

Original PR: #1766

[mssonicbld]

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