Re-enable PropertyOverridesContainSemicolon regression test

This test does a full C# build internally to guard against regression of an old bug, but that currently fails (on my machine at least) with:

```
O:\msbuild\bin\Windows_NT\Debug-NetCore\Microsoft.Common.CurrentVersion.targets(1822,5): error MSB3248: Parameter "AssemblyFiles" has invalid value "C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\mscorlib.dll". Could not load file or assembly 'System.Reflection.Metadata, Version=1.1.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. The located assembly's manifest definition does not match the assembly reference. (Exception from HRESULT: 0x80131040)
Done executing task "ResolveAssemblyReference" -- FAILED.
```

Which causes the test to fail, because it expected success. This doesn't appear to be the problem that's being tested for, so I'm going to disable the test until we can figure out its root cause.