Fix container parsing for k8s cgroup

[delner]

Adds support for:

• k8s burstable containers (drops .slice and .scope from IDs)
• Gnome VTE cgroup
• More defensive behavior against empty cgroup paths.

Also includes some minor refactoring for tests and a slightly more permissive expectation (so it doesn't break with pry.)

[delner]

Before merging this, I want to investigate a report for Fargate 1.4... if there's a change to be made, it'd be easier to add it to this PR.

[codecov-commenter]

Codecov Report

Merging #1487 (e8226ce) into master (480ebe2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1487   +/-   ##
=======================================
  Coverage   98.23%   98.24%           
=======================================
  Files         863      863           
  Lines       41582    41681   +99     
=======================================
+ Hits        40849    40948   +99     
  Misses        733      733           

Impacted Files	Coverage Δ
lib/ddtrace/runtime/container.rb	93.02% <100.00%> (+0.52%)	⬆️
spec/ddtrace/runtime/cgroup_spec.rb	100.00% <100.00%> (ø)
spec/ddtrace/runtime/container_spec.rb	100.00% <100.00%> (ø)
spec/support/container_helpers.rb	100.00% <100.00%> (ø)

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[ivoanjo]

@delner not sure if it's helpful, but I have a repo https://github.com/DataDog/profile-ruby-app-on-amazon-ecs-experiment (internal link, sorry!) with some scaffolding to deploy stuff to ecs/eks, it may come in handy.

[delner]

Okay, fixed one more scenario regarding Fargate 1.4+ cgroups. This should be ready for review.