Escape skip reason in junitxml

[clee2000]

Escape invalid characters in the skip message in junitxml.

Small repro:

import pytest

class TestA:
    @pytest.mark.skip("\33[31;1madsfsd\33[0m")
    def test_a(self):
        assert 2 == 2

Run the above via pytest repro.py --junit-xml=repro.xml
The result is (formatted for easier reading):

<?xml version="1.0" encoding="utf-8"?>
<testsuites>
    <testsuite name="pytest" errors="0" failures="0" skipped="1" tests="1" time="0.045"
        timestamp="2024-01-18T09:38:22.499385" hostname="csl-mbp">
        <testcase classname="repro.TestA" name="test_a" time="0.000">
            <skipped type="pytest.skip" message="�[31;1mrandom colored skip reason�[0m">/Users/csl/zzzzzzzz/random-testing/repro.py:4:
                #x1B[31;1mrandom colored skip reason#x1B[0m</skipped>
        </testcase>
    </testsuite>
</testsuites>

There's an invalid XML character (Unicode 0x1b) in the message that prevents me from parsing the XML.
The output after this change is:

<?xml version="1.0" encoding="utf-8"?>
<testsuites>
    <testsuite name="pytest" errors="0" failures="0" skipped="1" tests="1" time="0.037"
        timestamp="2024-01-18T09:39:55.805037" hostname="csl-mbp">
        <testcase classname="repro.TestA" name="test_a" time="0.000">
            <skipped type="pytest.skip" message="#x1B[31;1mrandom colored skip reason#x1B[0m">/Users/csl/zzzzzzzz/random-testing/repro.py:4:
                #x1B[31;1mrandom colored skip reason#x1B[0m</skipped>
        </testcase>
    </testsuite>
</testsuites>

[nicoddemus]

Awesome, thanks for the PR @clee2000!

Could you please add a test to https://github.com/pytest-dev/pytest/blob/main/testing/test_junitxml.py for completeness and to avoid a future regression?

[nicoddemus]

Thanks a lot @clee2000, appreciate it!