[SPARK-35263][TEST] Refactor ShuffleBlockFetcherIteratorSuite to reduce duplicated code

### What changes were proposed in this pull request?
Introduce new shared methods to `ShuffleBlockFetcherIteratorSuite` to replace copy-pasted code. Use modern, Scala-like Mockito `Answer` syntax.

### Why are the changes needed?
`ShuffleFetcherBlockIteratorSuite` has tons of duplicate code, like https://github.com/apache/spark/blob/0494dc90af48ce7da0625485a4dc6917a244d580/core/src/test/scala/org/apache/spark/storage/ShuffleBlockFetcherIteratorSuite.scala#L172-L185 . It's challenging to tell what the interesting parts are vs. what is just being set to some default/unused value.

Similarly but not as bad, there are many calls like the following
```
verify(transfer, times(1)).fetchBlocks(any(), any(), any(), any(), any(), any())
when(transfer.fetchBlocks(any(), any(), any(), any(), any(), any())).thenAnswer ...
```

These changes result in about 10% reduction in both lines and characters in the file:
```bash
# Before
> wc core/src/test/scala/org/apache/spark/storage/ShuffleBlockFetcherIteratorSuite.scala
    1063    3950   43201 core/src/test/scala/org/apache/spark/storage/ShuffleBlockFetcherIteratorSuite.scala

# After
> wc core/src/test/scala/org/apache/spark/storage/ShuffleBlockFetcherIteratorSuite.scala
     928    3609   39053 core/src/test/scala/org/apache/spark/storage/ShuffleBlockFetcherIteratorSuite.scala
```

It also helps readability, e.g.:
```
    val iterator = createShuffleBlockIteratorWithDefaults(
      transfer,
      blocksByAddress,
      maxBytesInFlight = 1000L
    )
```
Now I can clearly tell that `maxBytesInFlight` is the main parameter we're interested in here.

### Does this PR introduce _any_ user-facing change?
No, test only. There aren't even any behavior changes, just refactoring.

### How was this patch tested?
Unit tests pass.

Closes apache#32389 from xkrogen/xkrogen-spark-35263-refactor-shuffleblockfetcheriteratorsuite.

Authored-by: Erik Krogen <[email protected]>
Signed-off-by: Mridul Muralidharan <mridul<at>gmail.com>

Author: xkrogen