feat(test): capture stdout/stderr in JUnit reporter

[lunadogbot]

Closes #20152.

Problem

The JUnit reporter dropped all test output. The captured stdout/stderr arrived as a single global stream with no way to tell which test or step produced it, so report_output was a no-op with a TODO.

Solution

Within a worker, tests run sequentially, so output between a test's Wait and Result belongs to that test — and the channel's existing stdio-sync mechanism already guarantees correct interleaving of output and lifecycle events. The LSP reporter already relies on this to attribute output to a current_test; this PR applies the same approach to the JUnit reporter.

• Tag output with its stream. TestEvent::Output(Vec<u8>) becomes TestEvent::Output(TestStdioStream, Vec<u8>), where the new TestStdioStream enum distinguishes Stdout/Stderr. Each TestStream now records which stream it reads from.
• Track the active test/step in the JUnit reporter. current_test is set on report_wait/report_step_wait and restored/cleared on the corresponding results (mirroring the LSP reporter's stack-like handling), so step output is attributed to the step and pre/post-step output to the parent test.
• Emit the output. Captured bytes are appended (ANSI-stripped, as the JUnit reporter already strips color) to per-case system_out/system_err buffers and serialized as <system-out>/<system-err> elements — matching the convention used by other JUnit producers such as Vitest.
• Updated the report_output trait signature across all reporters; pretty/dot/tap/compound/LSP ignore the stream and keep their prior behavior.

Example

Deno.test("captures stdout and stderr", () => {
  console.log("log line");
  console.error("error line");
});

<testcase name="captures stdout and stderr" classname="./main.ts" time="0.019" line="1" col="6">
    <system-out>log line
</system-out>
    <system-err>error line
</system-err>
</testcase>

Tests

• New spec test junit_capture_output covering stdout, stderr, and per-step attribution.
• Updated junit and junit_multiple_test_files .out files, whose existing console.log/console.error tests now show captured output.
• Verified hide_stacktraces, junit_nested, junit_console_formatting, and the junit_path integration test are unaffected.

https://claude.ai/code/session_013BQKi6tsSyAeFT3UsBgAfR

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