Make cut_ocaml_deps tests more robust and debuggable

Summary:
## Background

This is [a tool for build speed optimization](https://fburl.com/cut_ocaml_dependencies)
The tests sometimes failed in CI.

## Changes

Robustness:
- The tool `buck bxl` to query the build graph, which can be problematic when the tool itself is invoked via buck (`buck test`). We update to build in randomly-generated temporary isolation directories so the tests can be parallelized.

Debuggability:
- We now show stderr on failure. When the tool is run in test mode, it suppresses stderr unless there are failures: this enables deterministic snapshotting

Differential Revision: D88394962

fbshipit-source-id: 71a090e2a9fb5f0e1e85230216b0a23184dc64e9

Author: mheiber

hphp/hack/test/scripts/cut_ocaml_dependencies/@dot1.input

@@ -1 +1 @@
---output dot --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:a --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:c --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --buck-isolation-dir isolation_dir
+--output dot --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:a --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:c --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --from-test --from-buck

hphp/hack/test/scripts/cut_ocaml_dependencies/@dot2.input

@@ -1 +1 @@
---output dot --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:d --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:c --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --buck-isolation-dir isolation_dir
+--output dot --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:d --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures:c --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --from-test --from-buck

hphp/hack/test/scripts/cut_ocaml_dependencies/@unused1.input

@@ -1 +1 @@
---output unused --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/b:b --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --buck-isolation-dir isolation_dir
+--output unused --from-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/b:b --to-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --project-targets-pattern //hphp/hack/test/scripts/cut_ocaml_dependencies/fixtures/... --buck-mode @//mode/dev-nosan-lg --from-test --from-buck