Modernize C-style array to std::array in NetworkBandwidthEstimator tests

[Copilot]

Addresses review feedback from PR #808 to replace C-style array patterns with modern C++ idioms in FunctionalTests.cpp.

Changes

• Replace C-style array with std::array<TestData, 8> for compile-time size tracking
• Replace sizeof(test_data)/sizeof(test_data[0]) with .size() method call
• Change loop iterator from int to size_t for type consistency
• Extract anonymous struct to named TestData struct for clarity

Before:

const struct {
    double now;
    // ... fields
} test_data[] = { /* data */ };

for(int i=1; i<sizeof(test_data)/sizeof(test_data[0]); i++)

After:

struct TestData {
    double now;
    // ... fields
};
const std::array<TestData, 8> test_data{{ /* data */ }};

for(size_t i=1; i<test_data.size(); i++)

Maintains C++11 compatibility as required by the test suite build configuration.

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