Fix the "valid.xml" OpenCover test case

[Tim-Pohlmann]

Fixes #4038

Additional context: From my understanding, the problem is that valid.xml was created with a very old version of OpenCover, which did not have sl and offsetend attributes in Branchpoints. I did not go through the trouble of fully recreating the file; instead, I just added the missing attributes with arbitrary values.

[sonarcloud[bot]]

Quality Gate passed for 'Sonar .NET Java Plugin'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate passed for 'SonarAnalyzer for .NET'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[Tim-Pohlmann]

The value/effort ratio does not make this a great fit for the current sprint. I will drop this.