add_dune_test: Handle complex expressions in `CMAKE_GUARD`

Ansgar Burchardtrequested to merge
bugfix/complex-CMAKE_GUARD-expressions into master

The check if(NOT ${condition}) does not behave as expected when condition is a complex string expression such as A OR B. It does however work when condition is a list.

Therefore convert condition to a list by calling separate_arguments.

Note that this change also adds missing parenthesis around condition which also results in wrong results for complex expressions.

Closes issue #39 (closed).

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