Auto merge of #84568 - andoriyu:libtest/junit_formatter, r=yaahc

feat(libtest): Add JUnit formatter

tracking issue: https://github.com/rust-lang/rust/issues/85563

Add an alternative formatter to `libtest`. Formatter produces valid xml that later can be interpreted as JUnit report.

Caveats:

- `timestamp` is required by schema, but every viewer/parser ignores it. Attribute is not set to avoid depending on chrono;
- Running all "suits" (unit tests, doc-tests and integration tests) will produce a mess;
- I couldn't find a way to get integration test binary name, so it's just goes by "integration";

Sample output for unit tests (pretty printed by 3rd party tool):
```
<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="test" package="test" id="0" errors="0" failures="0" tests="13" skipped="1">
    <testcase classname="results::tests" name="test_completed_bad" time="0"/>
    <testcase classname="results::tests" name="suite_started" time="0"/>
    <testcase classname="results::tests" name="suite_ended_ok" time="0"/>
    <testcase classname="results::tests" name="suite_ended_bad" time="0"/>
    <testcase classname="junit::tests" name="test_failed_output" time="0"/>
    <testcase classname="junit::tests" name="test_simple_output" time="0"/>
    <testcase classname="junit::tests" name="test_multiple_outputs" time="0"/>
    <testcase classname="results::tests" name="test_completed_ok" time="0"/>
    <testcase classname="results::tests" name="test_stared" time="0"/>
    <testcase classname="junit::tests" name="test_generate_xml_no_error_single_testsuite" time="0"/>
    <testcase classname="results::tests" name="test_simple_output" time="0"/>
    <testcase classname="test" name="should_panic" time="0"/>
    <system-out/>
    <system-err/>
  </testsuite>
</testsuites>
```

Sample output for integration tests (pretty printed by 3rd party tool):

```
<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="test" package="test" id="0" errors="0" failures="0" tests="1" skipped="0">
    <testcase classname="integration" name="test_add" time="0"/>
    <system-out/>
    <system-err/>
  </testsuite>
</testsuites>
```

Sample output for Doc-tests (pretty printed by 3rd party tool):

```
<?xml version="1.0" encoding="UTF-8"?>
<testsuites>
  <testsuite name="test" package="test" id="0" errors="0" failures="0" tests="1" skipped="0">
    <testcase classname="src/lib.rs" name="(line 2)" time="0"/>
    <system-out/>
    <system-err/>
  </testsuite>
</testsuites>
```

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