feat: emit per-test JUnit XML from the clojure_test runner

[english]

Summary

The clojure_test runner only reported a pass/fail exit code, so Bazel
generated a single stub JUnit result per target. The runner now emits structured
JUnit XML to $XML_OUTPUT_FILE, so Bazel surfaces one result per deftest
with timing and failure/error detail instead of a synthesised stub.

What changed

• testrunner.clj hooks clojure.test's :begin-test-var / :end-test-var
  events to build one <testcase> per deftest (grouped into one
  <testsuite> per namespace), then serialises to $XML_OUTPUT_FILE.
• Assertion failures → <failure>, uncaught exceptions → <error>, each with
  message + expected/actual or stacktrace.
• Namespace-load failures and fixture exceptions surface as errored testcases
  rather than disappearing into the exit code.
• XML is hand-rolled (no new deps) so the isolated AOT bootstrap keeps working;
  console output is unchanged.

In addition to producing the JUnit report, running bazel test with the --test_summary=detailed flag will have bazel output per-deftest results, including timing, like:

//test:ci_failure_demo_test.test                                         FAILED in 6.5s
    PASSED  ci-failure-demo-test.slow-demo (5.0s)
    PASSED  ci-failure-demo-test.passing-demo (0.0s)
    FAILED  ci-failure-demo-test.assertion-failure-demo (0.0s)
    ERROR   ci-failure-demo-test.uncaught-error-demo (0.0s)
    FAILED  ci-failure-demo-test.multiple-failures-demo (0.0s)
    FAILED  ci-failure-demo-test.property-failure-demo (0.0s)

Notes

• <system-out> is deliberately omitted. The old path wrote no
  $XML_OUTPUT_FILE, so Bazel's stub generator
  (generate-xml.sh)
  embedded the full test.log in <system-out>; emitting our own XML drops
  that auto-embedded log. This matches Bazel's own
  AntXmlResultWriter
  (which leaves <system-out> empty), and the console log is still captured in
  test.log.