Record app_path in Junit Reports (RDT-1098)

[hfudev]

terms:

• main junit report -> the junit file generated by --junitxml
• dut junit reports -> the junit files generated for multi-dut tests dut-0.xml, dut-1.xml, ...

For single-dut test cases, we shall record the app_path into the main junit report

For multi-dut test cases, we shall first record them correctly into their own dut junit reports, then use the JunitMerger to copy those to the main junit report. For example,

• dut[0].app_path -> app_path[0] -> dut-0.xml
• dut[1].app_path -> app_path[1] -> dut-1.xml

for app_path, it's better to use relative path to the pytest rootdir.