Add user_properties to test case report

[kurt-cb]

instead of creating a new API for adding properties, capture pytest user_properties. This removes the need for a special fixture and also works correctly with pytest-parallel

Setting internal variables with the record_nunit_property is non-functional in pytest-parallel as the modifications are done in a different process (nunit_xml is a sub-process copy, so the changes are lost). user_properties are reported and re-synchronized with the main process via the test_report. This is a better location for test case properties IMHO.

[kurt-cb]

steps to reproduce:

1. create a test case that uses record_nunit_property to set "TEST" to "123" and record_property to set "TEST2" to "ABC"
2. run test with pytest-parallel enabled for 1 cpu
3. examine test-output.xml and notice that "TEST" property with "123" is missing (due to the update in a child process) and "TEST2" is set to "ABC" as it was marshaled to the main process in the test_report's user_properties.

[kurt-cb]

this PR addresses #60 by updating the properties from the standard record_property builtin fixture.