Use xunit1 as default junit family version (sonic-net#10491)

What is the motivation for this PR?
When we use python3 env to run tests, it would use latest pytest by default, with junit_family=xunit2, which would not generate "file" and "line" field in test result xml file

How did you do it?
Use junit_family=xunit1 by default in pytest.ini file

How did you verify/test it?
Run test in python2 and python3 env, difference is only "file" and "line" fields in xml file

tests/pytest.ini

@@ -1,4 +1,5 @@
 [pytest]
+junit_family=xunit1
 markers:
     acl: ACL tests
     bsl: BSL tests