triage tool: manage test case output files

[olupton]

The main feature added here is that a special directory, /triage-tool-output, is mounted into the container when the test case is executed, and the host-side versions of this directory are tracked by the tool so it can identify those corresponding to the last-known-good and first-known-bad invocations of the test case.

This means that the user can immediately examine "what changed", without having to manually re-run those two configurations.

Other minor changes:

• Add a Container interface with PyxisContainer and DockerContainer implementations, for neater type annotations.
• Add a result cache, which allows the tool to avoid some redundant re-executions
• Write the same log information to info.log as to the console
• Only call git fetch if needed; if commit-level triage is using a "new"/failing container, all relevant git history should already be available.

[gspschmid]

LGTM, minor comments