Add a test framework for monitoring the host system from inside a container

[fearful-symmetry]

What does this PR do?

Closes elastic/beats#38241

This adds a lightweight test framework that runs a set of system tests under a container with the goal of monitoring the host system. The goal with these tests is to catch the numerous edge cases that happen when the system metrics function from a /hostfs path inside a container.

The tests have a fairly large matrix of configurations, as we need to test both a wide variety of container permission settings, as well as differences in how linux distros will configure cgroups.

The framework here was designed with the goal of being relatively idiomatic; you can just run the framework with go test as you would normally.

You can run the tests yourself with go test -v ./tests

As you may have noticed, there's a non-zero amount of TODO statements here, since these tests were built to aggravate a bunch of existing bugs, so certain parts of the tests will remain un-implemented until those bugs are fixed.

Why is it important?

See elastic/beats#38241, we really need test for this particular case.

List of bugs that are responsible for TODO statements in the tests:

• avoidFileSystem() needs to handle an alternate rootfs #141
• error checks when reading /proc/[pid]/exe don't account for kernel processes or docker permissions shenanigans  #135
• SubsystemMountpoints() can return wong V2 path for non-root container environments #139
• cgroups not accounting for relative paths that happen when reading from a container mount #132
• Make the "host FS" location configurable go-sysinfo#12

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.md

[belimawr]

Do we need to keep those here?

Was it left over from development or is it likely to be needed?

[fearful-symmetry]

Depending on how creative I get with the buildkite images later, I might need it, so I'm leaving it in

[faec]

Looks good!