Port tests of other PR #6
Description
Port the tests (jabgui/src/test/java/org/jabref/gui/mergebibfilesintocurrentbib/MergeBibFilesIntoCurrentBibTest.java) - if there are more test cases
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Port the tests (jabgui/src/test/java/org/jabref/gui/mergebibfilesintocurrentbib/MergeBibFilesIntoCurrentBibTest.java) - if there are more test cases