Make test.sus_regression.sh's output environment-invariant

[VonTum]

Right now, my home directory is encoded in the output files from test.sus_regression.sh. That's because the errors the compiler prints include the file paths for the source files.

• Compiler flag "--ci-mode" that omits file paths, or makes them more uniform in some way, such that different machines will still output the same text.
• Integrate in CI. Error if there's a diff between test.sus_output/errors.txt and the committed version