Add shrink scenario to PRs and bump robotest to 2.0.0.

[wadells]

Description

This change adds a shrink scenario to PR builds and updates to a more recent robotest.

Significant changes between uid-gid & 2.0.0 include:

• shrink testing is supported
• robotest has smarter assertions about gravity status
• robotest now follows semver
• version & commit information is logged
• Many other internal bugfixes

Type of change

• New feature (non-breaking change which adds functionality)
• Internal change (not necessarily a bug fix or a new feature)

Linked tickets and other PRs

This is a port of #1670 and #1646.

TODOs

• Self-review the change
• Address review feedback

Testing done

See the robotest PRs linked in #1670 for pretty extensive testing related to each individual change. I did not do any 5.5.x specific testing. The PR build here serves as validation for this work.

[wadells]

Looks like gravitational/robotest#233 is incompatible with 5.5:

ERROR REPORT:
Original Error: *json.UnmarshalTypeError json: cannot unmarshal string into Go struct field ClusterStatus.system_status of type int
Stack Trace:
	/go/src/github.com/gravitational/robotest/infra/gravity/parse.go:19 github.com/gravitational/robotest/infra/gravity.parseStatus.func1
	/go/src/github.com/gravitational/robotest/lib/ssh/run_parse.go:103 github.com/gravitational/robotest/lib/ssh.RunAndParse.func1
	/go/src/runtime/asm_amd64.s:1337 runtime.goexit

Tracking this as gravitational/robotest#247. Closing this PR for now. I'll return to it once I've got a 2.0.x suitable for 5.5.