added test for prodnote mapping issue in cover
added test for prodnote mapping issue in cover

fixes #151 as of PR #205 (this commit adds tests)

mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,nlbdev/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator,mtmse/nordic-epub3-dtbook-migrator