python3-et_xmlfile: add python3-xml dep, fix test

python3-et_xmlfile: add python3-xml dep, fix test

The test.sh imports xml.etree.ElementTree from the stdlib, and
et_xmlfile itself falls back to stdlib xml when lxml is unavailable.
Add python3-xml to DEPENDS to ensure the xml module is present at
both install and test time.

The test used et_xmlfile.__version__ without importing the et_xmlfile
module (only 'from et_xmlfile import xmlfile' was present). Add
'import et_xmlfile' so the version check works correctly.

The test.sh lacked a package name guard, causing it to run (and fail)
when the -src subpackage was tested without python3 installed.
Add the standard guard to skip the test for non-main packages.

Signed-off-by: Alexandru Ardelean <redacted>