buck2_test: don't poison results channel via internal orchestrator drop

[vlad-ivanov-name]

Any buck2 test run in which at least one target used InternalRunnerTestInfo fails at teardown with

Did not receive all results from executor: BuckTestOrchestrator
exited before end-of-tests was received

even though all per-test results were already reported correctly.

Root cause: drop order in test_targets.

Fix: drop the internal orchestrator after try_fold.

This fix also surfaces other issue so the next fix is the exit code for failing internal-runner tests: they showed Fail in the UI but buck2 test exited 0 (green CI on red tests).

Reproducer

.buckconfig

[cells]
root = .

defs.bzl

def _listing(content: str) -> list[dict]:
    return [{"name": n, "filter": n} for n in content.split()]

def _result(stdout: str, stderr: str, exit_code: int) -> list[dict]:
    # empty -> the runner synthesizes PASS/FAIL from the exit code
    return []

def _impl(ctx):
    return [DefaultInfo(), InternalRunnerTestInfo(
        type = "sh",
        listing_command = ["/bin/sh", "-c", "echo alpha beta"],
        command = ["/bin/sh", "-c", "exit 0"],
        parse_test_listing = _listing,
        parse_test_result = _result,
    )]

internal_test = rule(impl = _impl, attrs = {})

BUCK

load(":defs.bzl", "internal_test")

internal_test(name = "t")

Before

$ buck2 test //:t; echo $?
✓ Pass: alpha (0.0s)
✓ Pass: beta (0.0s)
Command failed:
Test executor exited unexpectedly with status 1.
Stdout:

Stderr:
Command failed: code: 'Unknown error', message: "Failed to report end-of-tests: end_of_tests was received twice (internal error)"
1

After

$ buck2 test //:t; echo $?
Tests finished: Pass 2. Fail 0. Timeout 0. Fatal 0. Skip 0. Omit 0. Infra Failure 0. Build failure 0
0

With a failing command (exit 1 for test beta):

$ buck2 test //:t; echo $?
Tests finished: Pass 1. Fail 1. Timeout 0. Fatal 0. Skip 0. Omit 0. Infra Failure 0. Build failure 0
1 TESTS FAILED
  ✗ beta (<unspecified>)
32

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D115868652. (Because this pull request was imported automatically, there will not be any future comments.)

[vlad-ivanov-name]

Turns out this requires some adjustments in how exit code is derived. I will fix that and mark it ready for review after.

Edit: fixed