Fix the "valid.xml" OpenCover test case

[andrei-epure-sonarsource]

The current test case for a valid OpenCover report ("valid.xml") creates noise when being parsed.

OpenCover parser: invalid start line for file (ID '3', path 'MyLibrary\Adder.cs', indexed as ...

This should not happen for valid XML. We should regenerate this test case. But bear in mind the OpenCover versioning. We need to use also an older open cover format which doesn't have the "fileid" in the SequencePoint attribute. Otherwise, we'll lose coverage.

Alternatively, we can decide to drop this "RefFile" UID fallback feature - must investigate in which OpenCover version it got dropped (if in any).

[Tim-Pohlmann]

I updated priority and effort and moved this back to the backlog.

[pavel-mikula-sonarsource]

Moved to NET-1926