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

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:

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

While on 10.0.10, it's:

```c#
[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. https://github.com/dotnet/runtime/pull/119280, which bumps version to 9.0.10, had to manually fix the test and https://github.com/dotnet/runtime/pull/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.