[DirectX][ObjectYAML] Attempt to fix flaky PRIVPart.yaml (#206278)
This test was meant to round-trip YAML twice, to ensure correct
processing of non-4-byte-padded PRIV section.
However, second invocation of yaml2obj had wrong arguments (it was
reading from test file instead of stdin). Fix that.
Also, round-trips were split into several RUN lines, to make it clear on
which line an error occurs if the test is still flaky.