ImporterTests.Import is too brittle and breaks when runtime patch version grows past 10

[svick]

The test System.Runtime.Serialization.Schema.Tests.ImporterTests.Import checks the exact number of characters in the generated C# code. This is a problem, because the generated code on .Net 10.0.9 contains:

[System.CodeDom.Compiler.GeneratedCodeAttribute("System.Runtime.Serialization.Schema", "10.0.0.9")]

While on 10.0.10, it's:

[System.CodeDom.Compiler.GeneratedCodeAttribute("System.Runtime.Serialization.Schema", "10.0.0.10")]

This means the number of characters in the generated code changes whenever the patch version reaches 10. #119280, which bumps version to 9.0.10, had to manually fix the test and #128941, which bumps version to 10.0.10 will likely have to do the same.

I think the test should be changed, so that it's not sensitive to version changes.

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