Execute benchmarks as current user

[smarr]

The main goal of the PR is to ensure that benchmarks are run with the current user.
Before, when we use denoise, we'd end up running with root. Now, we return back to the initial user and execute the benchmarks as that one.

The core change is thus, to add a sudo -u $currentUser to the command line that executes the benchmark.
Though to make this work, we need a few more changes:

• use env -i to create an empty environment in $currentUser, and only set the vars we want
• pass command line as a list of strings instead of a single string interpreted by a shell. This is needed to make sure the env var values are not wrongly escaped
• I renamed cmdline to cmd in places where it's now a list[str]. We keep it as a str until all ReBench variables are replaced during preparing the cmdline when creating the runs.
• fix how we determine the current user. Use geteuid() and getpwuid() to avoid picking up stale env vars with the users name. This was an issue on macOS.

Added tests:

• end-to-end check of env vars a benchmark sees
• end-to-end check of which user the benchmark is executed with
• end-to-end integration test, which checks rebench, and our denoise abilities, too (note, the results are currently machine specific, and will need to be updated over time)

@flohuemer, since you proposed the initial change, could you give this a read/review? Thanks!