Improve test coverage

[pohlm01]

There are a few files that are not yet properly covered by test.

• openleadr-vtn/src/data_source/postgres/report.rs
• openleadr-vtn/src/data_source/postgres/resource.rs
• openleadr-vtn/src/data_source/postgres/user.rs
• openleadr-client/tests/report.rs (file does not exist yet, use /tests/ven.rs as inspiration)

Codecov provides a nice overview as well and might show files that I missed in the above list.
Any contribution improving the test coverage is welcome, even if it only adds a (small) part.