[java-firestore] firestore.it.ITQueryWatchTest: nonEmptyResults_modifiedDocument_REMOVED failed

Note: googleapis/java-firestore#1010 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

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commit: 25edee4fca0994aea96867f2a63365238d617b36
buildURL: [Build Status](https://source.cloud.google.com/results/invocations/733c716b-fc70-4356-bf47-62fdc8c284ed), [Sponge](http://sponge2/733c716b-fc70-4356-bf47-62fdc8c284ed)
status: failed
Test output

events[

event{error=null,value={documents[{path=java-nonEmptyResults_modifiedDocument_REMOVED-Y06F1dorBSYSTSCT7ZYg/doc,data={foo=bar}}],documentChanges[{path=java-nonEmptyResults_modifiedDocument_REMOVED-Y06F1dorBSYSTSCT7ZYg/doc,data={foo=bar}}]}},

]

value of               : size()

expected to be in range: [2..2]

but was                : 1

	at com.google.cloud.firestore.it.ITQueryWatchTest$QuerySnapshotEventListener$ListenerAssertions.eventCountIsAnyOf(ITQueryWatchTest.java:809)

	at com.google.cloud.firestore.it.ITQueryWatchTest.nonEmptyResults_modifiedDocument_REMOVED(ITQueryWatchTest.java:413)

从 ITQueryWatchTest.java 开始，重点查看第 413 行的 nonEmptyResults_modifiedDocument_REMOVED 和第 809 行的 eventCountIsAnyOf。查看失败构建的输出以及相关的 googleapis/java-firestore#1010 issue，以了解为什么只观察到了一个事件。理解测试行为并使集成测试可靠通过后，即视为完成。

由索引模型根据 Issue 内容生成。