[Shell] Fix IDE building projects wrong

This fixes IDE building projects which are not built in the current
configuration.

The error is different from msbuild, because this says, when GuiUnit isn't built:

Using MSBuild:
The type or namespace name 'TextfixtureSetUpAttribute' could not be found.

Using MD:
CSC: Error CS0006: Metadata file 'F:\...\bin\net_4_5\GuiUnit.exe' could not be found (CS0006) (...)