https://searchfox.org/mozilla-central/rev/e3fc8f8970491aef14d3212b2d052942f4d29818/dom/base/test/gtest/TestPlainTextSerializer.cpp#113 doesn't set the "mozquote" attribute. However, https://searchfox.org/mozilla-central/rev/e3fc8f8970491aef14d3212b2d052942f4d29818/editor/libeditor/HTMLEditorDataTransfer.cpp#2188 sets that attribute. Which is the case intended to be tested here.
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https://searchfox.org/mozilla-central/rev/e3fc8f8970491aef14d3212b2d052942f4d29818/dom/base/test/gtest/TestPlainTextSerializer.cpp#113 doesn't set the "mozquote" attribute. However, https://searchfox.org/mozilla-central/rev/e3fc8f8970491aef14d3212b2d052942f4d29818/editor/libeditor/HTMLEditorDataTransfer.cpp#2188 set that attribute. Which is the case intended to be tested here.