Fixed flaky circuitBreaker unit test.

[nhtruong]

Here's the actual code in Transport.js:

    const MAX_STRING_LENGTH = buffer.constants.MAX_STRING_LENGTH;
    const HEAP_SIZE_LIMIT = require('v8').getHeapStatistics().heap_size_limit;
    ...
    const shouldApplyCircuitBreaker = (contentLength) => {
      if (!this.memoryCircuitBreaker || !this.memoryCircuitBreaker.enabled) return false;
      const maxPercentage = validateMemoryPercentage(this.memoryCircuitBreaker.maxPercentage);
      const heapUsed = process.memoryUsage().heapUsed;
      return contentLength + heapUsed > HEAP_SIZE_LIMIT * maxPercentage;
    };

However the current unit test sets the contentLength off of MAX_STRING_LENGTH instead of HEAP_SIZE_LIMIT. This results in the test failing on earlier versions of node.js where heapUsed is smaller and doesn't cause contentLength + heapUsed > HEAP_SIZE_LIMIT * maxPercentage to return true.

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