[dune_add_test()] Allow to attach labels to tests.

See #107.

This makes it possible to build and run only tests for a particular label. For instance, to only do "quick" tests:

make build_quick_tests
ctest -L '^quick$'

These labels can be used in ctest -L ${label_regex} to run only tests with certain labels.

Also, for each label there will be a custom target build_${label}_tests that builds all tests with that label. These targets can be added explicitly using dune_declare_test_label(labels...) so make build_foo_tests will succeed and do nothing if there are no tests with label "foo".

The label "quick" is always predeclared.

Intent-to-merge: 2018-03-28