Rename log.txt to screen-output in some tests

[anne-glerum]

Some tests have a log.txt reference file, but not a screen-output file. This PR renames the log.txt to screen-output.

For all pull requests:

• I have followed the instructions for indenting my code.

For new features/models or changes of existing features:

• I have tested my new feature locally to ensure it is correct.
• I have created a testcase for the new feature/benchmark in the tests/ directory.
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[gassmoeller]

/rebuild

[gassmoeller]

Interesting, I could have sworn our tester woud complain if it doesnt find a screen-output file (I think that happened to me when adding new tests). It looks like we need to take another look at our tester infrastructure, I opened #6998 to keep track of this.

This PR makes sense of course, lets see if the tester is now failing for these tests because it actually compares the output to the correct file.

[gassmoeller]

Hm, interesting. It seems the tester was running these tests already, even before this PR

(see e.g. this output for this PR: https://github.com/geodynamics/aspect/actions/runs/26938387023/job/79473538940?pr=6996#step:5:4210

and this output for #6992: https://github.com/geodynamics/aspect/actions/runs/26771570828/job/78912126379?pr=6992#step:5:4212)

Either way, we should be consistent about the name of this file.

Eventually, we should just rename all screen-output files to log.txt and only use log.txt everywhere. The name screen-output is a leftover from very early in ASPECTs development.