feat(cli): Adding JUnit test reports (#19747)

This commit makes the following changes
- Created a `CompoundTestReporter` to allow us to use multiple reporters
- Implements `JUnitTestReporter` which writes JUnit XML to a path
- Added a CLI flag/option `--junit` that enables JUnit reporting. By
default this writes the report to `stdout` (and disables pretty
reporting). If a path is provided, it will write the JUnit report to
that file while the pretty reporter writes to stdout like normal

Output of `deno -- test --allow-all --unstable
--location=http://js-unit-tests/foo/bar --junit
cli/tests/unit/testing_test.ts `
```xml
<?xml version="1.0" encoding="UTF-8"?>
<testsuites name="deno test" tests="7" failures="0" errors="0" time="0.176">
    <testsuite name="file:///Users/cooper/deno/deno/cli/tests/unit/testing_test.ts" tests="7" disabled="0" errors="0" failures="0">
        <testcase name="testWrongOverloads" time="0.012">
        </testcase>
        <testcase name="nameOfTestCaseCantBeEmpty" time="0.009">
        </testcase>
        <testcase name="invalidStepArguments" time="0.008">
        </testcase>
        <testcase name="nameOnTextContext" time="0.029">
            <properties>
                <property name="step[passed]" value="step ... nested step"/>
                <property name="step[passed]" value="step"/>
            </properties>
        </testcase>
        <testcase name="originOnTextContext" time="0.030">
            <properties>
                <property name="step[passed]" value="step ... nested step"/>
                <property name="step[passed]" value="step"/>
            </properties>
        </testcase>
        <testcase name="parentOnTextContext" time="0.030">
            <properties>
                <property name="step[passed]" value="step ... nested step"/>
                <property name="step[passed]" value="step"/>
            </properties>
        </testcase>
        <testcase name="explicit undefined for boolean options" time="0.009">
        </testcase>
    </testsuite>
</testsuites>
```

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