Replace deprecated usleep() with std::this_thread::sleep_for()

[Copilot]

Addresses review feedback on PR #1079 to modernize sleep calls in test code by replacing deprecated POSIX usleep() with standard C++11 threading primitives.

Changes:

• Replace all usleep() calls with std::this_thread::sleep_for(std::chrono::milliseconds())
• Remove <unistd.h> include (no longer needed)

Example:

// Before
usleep(100 * 1000);

// After
std::this_thread::sleep_for(std::chrono::milliseconds(100));

This aligns with the pattern already used in other test files (AampCurlDownloader/FunctionalTests.cpp, AampTsbMetaDataManagerTests, etc.) and improves portability by removing POSIX-specific dependencies.

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