Added tests for DatabaseFileLookup

[arfazhxss]

I have added a new test case directoryPathTests to the DatabaseFileLookupTest class. This test case verifies the functionality of the DatabaseFileLookup class in handling file paths and directories. It creates a temporary directory and file, sets up a BibDatabaseContext with two entries (one with a valid file path and one with an empty file path), and configures the FilePreferences to use the temporary directory as the default file directory. Then, it creates an instance of DatabaseFileLookup and asserts that the file in the temporary directory is found, while a non-existent file is not found. The test case checks if the FilePreferences are correctly set up with the temporary directory as the main file directory, and if the getPathOfDatabase method of DatabaseFileLookup returns an empty path.

Closes koppor#678

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
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