audio: Fix handling of relative XML include paths in VTS

Pass "no fixup base dirs" flag to the XInclude processor
to avoid modifications of the top-level elements from
included XML files as a result of "fixup."

Added tests to ensure that all relevant XInclude scenarios
work.

Bug: 192619060
Test: atest -host android.hardware.audio.common.test.utility_tests
Change-Id: Id595c9fd30be378d76387ee55a8937e0bf28d1cd