Fix JUnit XML generation.

[SanderVocke]

Problem:

• Incorrect JUnit XML
• In addition to this: the --out flag does nothing, as results always go to std::out and std::err.

Solution:

• Add layer
• "# tests" reported now maps to the amount of assertions
• Fix closing tag of
• Use output_filename to stream the report to.

Note:

• The adding of and tags as suggested in JUnit reporter produces invalid XML #567 was not done.

Testing:

• I tested this by passing a generated Junit file via the --out flag to https://github.com/marketplace/actions/junit-report-action (Github Actions Junit reporter). That reporter seems to be happy with the result now and happily generates reports (allthough not all corner cases were tested).

Issue: #567

Note: I am not well-versed in the modern C++ techniques used in boost-ext::ut. If the approach should be different it's probably best for someone to take over this PR.