Write tests to check that element functions can be called with input = output

[lukasdreyer]

According to documentation, most element functions should be able to be called with input = output. This can lead to some corner cases, see: https://github.com/DLR-AMR/t8code/pull/612 We should add a test, that checks that these functions give the same result when called this way

[ole-alb]

PR #1363 did something similar