How do you test a loop with TDD?

I often find that I am unsure how to test code that needs to loop. Take for example the following:

methodUnderTest(inputQueue) {
    while (inputQueue.count > 0) {
        process(inputQueue.dequeue());
    }
}

The first test might look like this:

processesInput1() {
   // Arrange
   inputQueue = [a];

   // Act
   methodUnderTest(inputQueue);

   // Assert
   Assert.wasProccessed(a);
}

The logical next test is:

processesInput2() {
   // Arrange
   inputQueue = [a, b];

   // Act
   methodUnderTest(inputQueue);

   // Assert
   Assert.wasProccessed(a);
   Assert.wasProccessed(b);
}

but by the time we get to:

processesInput3() {
   // Arrange
   inputQueue = [a, b, c];

   // Act
   methodUnderTest(inputQueue);

   // Assert
   Assert.wasProccessed(a);
   Assert.wasProccessed(b);
   Assert.wasProccessed(c);
}

It all starts to feel a little redundant. A good piece of advice I once heard about TDD was to treat tests like a specification. At what point does the test specify that an input of N items will all be processed? How best to portray this in a test?