🐛 Set a test-case ID from a JUnit <property>

[danwos]

The id of a test case shall able to be taken from the junit/json result data and not automatically generated.

This is needed it testfile and co are using :auto_cases:.

[chrisjsewell]

Transferred from useblocks/sphinx-test-reports#97 (originally titled “Allow ID to be set from test result data”).

Half of the original issue shipped: a JSON report can supply the ID today, because tr_json_mapping
takes an id key and the directive honours it (1.2.0, #99; the json_parser_custom fixture covers
it). JUnit XML cannot. A <property name="id" …> is parsed, but it only reaches the need if id
is listed in tr_extra_options, and doing that takes the build down with
TypeError: add_need() got multiple values for keyword argument 'id' from inside test_file.py
— measured at HEAD. Two things are wanted: a supported route from a JUnit property to the case ID
(and the same for test-reports build needs, which writes IDs of its own through identity.py),
and a guard so that tr_extra_options naming a reserved field is refused with a readable message
the way tr_file_option already is — fields.RESERVED_NAMES exists, check_field_name_collisions
just never looks at tr_extra_options.