Use BibTeX builder at `org.jabref.logic.importer.fileformat.BibtexImporterTest#importEntries`

[koppor]

(META: This issue is reserved for a university course. Please only work on it if it is part of your assignment)

JabRef offers a (kind of) Builder for BibTeX data:

    private static BibEntry titleSentenceCased = new BibEntry(StandardEntryType.Misc)
            .withCitationKey("title-sentence-cased")
            .withField(StandardField.TITLE, "Title Sentence Cased");

This builder is NOT used in org.jabref.logic.importer.fileformat.BibtexImporterTest#importEntries, but it should.

Task

Rewrite org.jabref.logic.importer.fileformat.BibtexImporterTest#importEntries to use assertEquals(list, bibEntries. The list, should be constructed with List.of(aksin, stdmodel, set, preissel2016). aksin, ... are BibEntrys created by new BibEntry... withField...)