dune_add_test: sets TIMEOUT to 300 by default

The documentation for dune_add_test states that TIMEOUT is an optional parameter overriding the default CMake timeout.

However the implementation just sets the timeout to 300s if no explicit timeout is given, no matter what the CMake default is.

Either the documentation should be updated and state that dune_add_test defaults to a 300s timeout (in which case TIMEOUT could be optional for MPI tests as there already is a default timeout), or the test's TIMEOUT property should only be set if an explicit TIMEOUT was given to dune_add_test.

@dominic: What do you think?