Adjusted old identifier export tests and added new ones checking for DOIs

.../java/at/tuwien/endpoint/PersistenceEndpointUnitTest.java

...st-service/src/test/resources/bibliography/style_apa4.txt

...service/src/test/resources/bibliography/style_bibtex0.txt

...service/src/test/resources/bibliography/style_bibtex1.txt

...service/src/test/resources/bibliography/style_bibtex2.txt

...service/src/test/resources/bibliography/style_bibtex3.txt

...t-service/src/test/resources/bibliography/style_ieee0.txt

...t-service/src/test/resources/bibliography/style_ieee1.txt

...t-service/src/test/resources/bibliography/style_ieee2.txt

...t-service/src/test/resources/bibliography/style_ieee3.txt

...tadata-db/test/src/main/java/at/tuwien/test/BaseTest.java