Aggregate junit report into one file

[gordonbondon]

Some systems require junit report to be in a single file. For example bazel https://bazel.build/reference/test-encyclopedia needs single file to be available in XML_OUTPUT_FILE path.

To avoid implementing junit aggregation in pkl wrappers in different places this PR instead adds a --junit-aggregate-reports flag to return all junit reports as a single file.

Additional flag --junit-aggregate-suite-name is added to allow overriding global test suite name from default pkl-tests