[ML] Remove use of XML serialisers in unit tests.

[edsavage]

Replacing the use of XML serialisers in unit tests with equivalent JSON serialisers allows the removal of dependencies on the 3rd party RapidXML library and also the bespoke XML parser and related code.

Labelling the PR as >non-issue as despite it's size it doesn't affect any external code or results.

Closes #2804

[valeriy42]

Good work! Does this mean that we can now remove libxml2 as our dependency? If so, we probably also need to adjust the CMake files and the 3rd party license info.

[valeriy42]

can this be a const reference?

[valeriy42]

params is not used in the function.

[edsavage]

Good work! Does this mean that we can now remove libxml2 as our dependency? If so, we probably also need to adjust the CMake files and the 3rd party license info.

I think so. My only hesitation is that it might be needed by one of the boost libraries. I'll check on all platforms before adjusting CMake, docs, build scripts etc.

[edsavage]

Good work! Does this mean that we can now remove libxml2 as our dependency? If so, we probably also need to adjust the CMake files and the 3rd party license info.

I think so. My only hesitation is that it might be needed by one of the boost libraries. I'll check on all platforms before adjusting CMake, docs, build scripts etc.

We still have a dependency on libxml2 because we're using it to generate XML formatted Boost Test results - see lib/test/CBoostTestXmlOutput.cc. I think we want to keep it as-is since it's much easier for a human to read than the JUNIT output we also generate (for the Buildkite Test Analytics)

[valeriy42]

LGTM. Also, thank you for including the SonarQube linting suggestions.

[elastic-sonarqube]

Quality Gate failed

Failed conditions
5.3% Duplication on New Code (required ≤ 3%)

See analysis details on SonarQube