pytest-pants: add JUnit XML output support, make test collector optional

[nprizal]

What

Adds JUnit XML output support to the pytest-pants runner and makes buildkite-test-collector optional.

Previously, pytest-pants always assumed JSON output from the collector. Now bktec detects the output format from the test command:

• --junit-xml={{resultPath}} → JUnit XML mode, no extra dependencies needed
• --json={{resultPath}} --merge-json → JSON mode, requires buildkite-test-collector in the pants resolve (existing behaviour, prints the existing info message)
• Neither flag → CommandNameAndArgs returns a clear error

The format is stored as a resultFormat string field on PytestPants instead of a useJUnit bool, so ResultFormat() now returns "junit", "json", or "".

Why

buildkite-test-collector can't be verified by bktec in the pants context (it lives inside the pants sandbox, not the system Python), so requiring it silently caused runtime failures. JUnit XML works out of the box without any extra setup.

Docs

Updated docs/pytest-pants.md to present JUnit as the default and the collector as an optional upgrade, following the structure of docs/pytest.md.

Closes TE-6055

[buildsworth-bk-app]

Solid, well-scoped change, and the structure mirrors the pytest runner closely.

One thing worth checking before merging: adding ResultFormat() turns on result uploads for pytest-pants that previously never happened (it used to fall through to the "" default in RunnerConfig). The JUnit case returning "junit" matches the nunit/gotest runners, but the JSON case returns "json" where the pytest runner returns "", so bktec would now upload the collector's JSON itself. I've left an inline question about whether that's intended or could double up with the collector's own upload.

Want to dig deeper? The full session log is attached to this Buildkite build. Download the session file and open a new pi session with it:

Download the buildsworth logs from build 508, then answer my questions about the findings.

[buildsworth-bk-app]

Question: This "json" value flows through ResultFormat() into uploadResults, so when BUILDKITE_TEST_ENGINE_UPLOAD_RESULTS is set, bktec will now upload the result file for the JSON path. The sibling pytest runner returns "" for its collector/JSON case (see pytest.go's ResultFormat()), deliberately leaving the upload to buildkite-test-collector, which sends results directly via BUILDKITE_ANALYTICS_TOKEN (set in buildCommand). Is the divergence here intentional — e.g. the collector doesn't receive the token inside the pants sandbox, so bktec has to do the upload — or could this end up uploading the same results twice?

[nprizal]

fixed in the latest commit.

[buildsworth-bk-app]

Non-blocking: These JUnit tests set resultFormat on the struct directly, so the new detection in NewPytestPants (--junit-xml -> "junit") isn't exercised end-to-end. The JSON branch is covered via NewPytestPants(...) in the existing tests, but the JUnit branch — the headline behaviour of this PR — isn't. A small test asserting that a --junit-xml={{resultPath}} command through NewPytestPants yields ResultFormat() == "junit" (and the .xml temp path) would lock that in cheaply.