UndefineProperties metadata is whitespace-sensitive

Reported by @jachin84 in https://github.com/Microsoft/msbuild/issues/3990#issuecomment-447823959

> Ok so this seems a little odd. I am using the following in a Directory.Build.props which fails every time because of concurrency issues.
>
> ```xml
>
>
>
> _PackageTempDir;AutoParameterizationWebConfigConnectionStrings
>
>
>
> ```
> If I do the following it work:
>
> ```xml
>
>
> _PackageTempDir;AutoParameterizationWebConfigConnectionStrings
>
>
> ```
> Not the only difference is splitting `UndefineProperties` on different lines.

This should ignore whitespace surrounding property names like other things do.

https://github.com/Microsoft/msbuild/blob/c4796604b9744d17cad2eaf028902af423c60e2f/src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs#L250-L260

Start in src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/MSBuild.cs at lines 250-260, where UndefineProperties is handled. Compare the multiline and single-line examples, then trace the property-name parsing. Done means surrounding whitespace is ignored consistently, with coverage added or updated for the reported multiline form.

Written by the indexing model from the issue text.